Chapter 12 Functions and Operators

Table of Contents

12.1 Function and Operator Reference
12.2 Type Conversion in Expression Evaluation
12.3 Operators
12.3.1 Operator Precedence
12.3.2 Comparison Functions and Operators
12.3.3 Logical Operators
12.3.4 Assignment Operators
12.4 Control Flow Functions
12.5 String Functions
12.5.1 String Comparison Functions
12.5.2 Regular Expressions
12.6 Numeric Functions and Operators
12.6.1 Arithmetic Operators
12.6.2 Mathematical Functions
12.7 Date and Time Functions
12.8 What Calendar Is Used By MySQL?
12.9 Full-Text Search Functions
12.9.1 Natural Language Full-Text Searches
12.9.2 Boolean Full-Text Searches
12.9.3 Full-Text Searches with Query Expansion
12.9.4 Full-Text Stopwords
12.9.5 Full-Text Restrictions
12.9.6 Fine-Tuning MySQL Full-Text Search
12.9.7 Adding a Collation for Full-Text Indexing
12.10 Cast Functions and Operators
12.11 XML Functions
12.12 Bit Functions
12.13 Encryption and Compression Functions
12.14 Information Functions
12.15 Spatial Analysis Functions
12.15.1 Spatial Function Reference
12.15.2 Argument Handling by Spatial Functions
12.15.3 Functions That Create Geometry Values from WKT Values
12.15.4 Functions That Create Geometry Values from WKB Values
12.15.5 MySQL-Specific Functions That Create Geometry Values
12.15.6 Geometry Format Conversion Functions
12.15.7 Geometry Property Functions
12.15.8 Spatial Operator Functions
12.15.9 Functions That Test Spatial Relations Between Geometry Objects
12.16 Miscellaneous Functions
12.17 Functions and Modifiers for Use with GROUP BY Clauses
12.17.1 GROUP BY (Aggregate) Functions
12.17.2 GROUP BY Modifiers
12.17.3 MySQL Handling of GROUP BY
12.18 Precision Math
12.18.1 Types of Numeric Values
12.18.2 DECIMAL Data Type Characteristics
12.18.3 Expression Handling
12.18.4 Rounding Behavior
12.18.5 Precision Math Examples

Expressions can be used at several points in SQL statements, such as in the ORDER BY or HAVING clauses of SELECT statements, in the WHERE clause of a SELECT, DELETE, or UPDATE statement, or in SET statements. Expressions can be written using literal values, column values, NULL, built-in functions, stored functions, user-defined functions, and operators. This chapter describes the functions and operators that are permitted for writing expressions in MySQL. Instructions for writing stored functions and user-defined functions are given in Section 20.2, “Using Stored Routines (Procedures and Functions)”, and Section 24.3, “Adding New Functions to MySQL”. See Section 9.2.4, “Function Name Parsing and Resolution”, for the rules describing how the server interprets references to different kinds of functions.

An expression that contains NULL always produces a NULL value unless otherwise indicated in the documentation for a particular function or operator.

Note

By default, there must be no whitespace between a function name and the parenthesis following it. This helps the MySQL parser distinguish between function calls and references to tables or columns that happen to have the same name as a function. However, spaces around function arguments are permitted.

You can tell the MySQL server to accept spaces after function names by starting it with the --sql-mode=IGNORE_SPACE option. (See Section 5.1.7, “Server SQL Modes”.) Individual client programs can request this behavior by using the CLIENT_IGNORE_SPACE option for mysql_real_connect(). In either case, all function names become reserved words.

For the sake of brevity, most examples in this chapter display the output from the mysql program in abbreviated form. Rather than showing examples in this format:

mysql> SELECT MOD(29,9);
+-----------+
| mod(29,9) |
+-----------+
|         2 |
+-----------+
1 rows in set (0.00 sec)

This format is used instead:

mysql> SELECT MOD(29,9);
        -> 2

12.1 Function and Operator Reference

Table 12.1 Functions/Operators

NameDescription
ABS()Return the absolute value
ACOS()Return the arc cosine
ADDDATE()Add time values (intervals) to a date value
ADDTIME()Add time
AES_DECRYPT()Decrypt using AES
AES_ENCRYPT()Encrypt using AES
AND, &&Logical AND
Area()Return Polygon or MultiPolygon area
AsBinary(), AsWKB()Convert from internal geometry format to WKB
ASCII()Return numeric value of left-most character
ASIN()Return the arc sine
=Assign a value (as part of a SET statement, or as part of the SET clause in an UPDATE statement)
:=Assign a value
AsText(), AsWKT()Convert from internal geometry format to WKT
ATAN2(), ATAN()Return the arc tangent of the two arguments
ATAN()Return the arc tangent
AVG()Return the average value of the argument
BENCHMARK()Repeatedly execute an expression
BETWEEN ... AND ...Check whether a value is within a range of values
BIN()Return a string containing binary representation of a number
BINARYCast a string to a binary string
BIT_AND()Return bitwise and
BIT_COUNT()Return the number of bits that are set
BIT_LENGTH()Return length of argument in bits
BIT_OR()Return bitwise or
BIT_XOR()Return bitwise xor
&Bitwise AND
~Invert bits
|Bitwise OR
^Bitwise XOR
CASECase operator
CAST()Cast a value as a certain type
CEIL()Return the smallest integer value not less than the argument
CEILING()Return the smallest integer value not less than the argument
Centroid()Return centroid as a point
CHAR_LENGTH()Return number of characters in argument
CHAR()Return the character for each integer passed
CHARACTER_LENGTH()Synonym for CHAR_LENGTH()
CHARSET()Return the character set of the argument
COALESCE()Return the first non-NULL argument
COERCIBILITY()Return the collation coercibility value of the string argument
COLLATION()Return the collation of the string argument
COMPRESS()Return result as a binary string
CONCAT_WS()Return concatenate with separator
CONCAT()Return concatenated string
CONNECTION_ID()Return the connection ID (thread ID) for the connection
Contains()Whether one geometry contains another
CONV()Convert numbers between different number bases
CONVERT_TZ()Convert from one timezone to another
CONVERT()Cast a value as a certain type
COS()Return the cosine
COT()Return the cotangent
COUNT(DISTINCT)Return the count of a number of different values
COUNT()Return a count of the number of rows returned
CRC32()Compute a cyclic redundancy check value
Crosses()Whether one geometry crosses another
CURDATE()Return the current date
CURRENT_DATE(), CURRENT_DATESynonyms for CURDATE()
CURRENT_TIME(), CURRENT_TIMESynonyms for CURTIME()
CURRENT_TIMESTAMP(), CURRENT_TIMESTAMPSynonyms for NOW()
CURRENT_USER(), CURRENT_USERThe authenticated user name and host name
CURTIME()Return the current time
DATABASE()Return the default (current) database name
DATE_ADD()Add time values (intervals) to a date value
DATE_FORMAT()Format date as specified
DATE_SUB()Subtract a time value (interval) from a date
DATE()Extract the date part of a date or datetime expression
DATEDIFF()Subtract two dates
DAY()Synonym for DAYOFMONTH()
DAYNAME()Return the name of the weekday
DAYOFMONTH()Return the day of the month (0-31)
DAYOFWEEK()Return the weekday index of the argument
DAYOFYEAR()Return the day of the year (1-366)
DECODE()Decodes a string encrypted using ENCODE()
DEFAULT()Return the default value for a table column
DEGREES()Convert radians to degrees
DES_DECRYPT() (deprecated 5.7.6)Decrypt a string
DES_ENCRYPT() (deprecated 5.7.6)Encrypt a string
Dimension()Dimension of geometry
Disjoint()Whether one geometry is disjoint from another
DIVInteger division
/Division operator
ELT()Return string at index number
ENCODE()Encode a string
ENCRYPT() (deprecated 5.7.6)Encrypt a string
EndPoint()End Point of LineString
Envelope()Return MBR of geometry
<=>NULL-safe equal to operator
=Equal operator
Equals()Whether one geometry is equal to another
EXP()Raise to the power of
EXPORT_SET()Return a string such that for every bit set in the value bits, you get an on string and for every unset bit, you get an off string
ExteriorRing()Return exterior ring of Polygon
EXTRACT()Extract part of a date
ExtractValue()Extracts a value from an XML string using XPath notation
FIELD()Return the index (position) of the first argument in the subsequent arguments
FIND_IN_SET()Return the index position of the first argument within the second argument
FLOOR()Return the largest integer value not greater than the argument
FORMAT()Return a number formatted to specified number of decimal places
FOUND_ROWS()For a SELECT with a LIMIT clause, the number of rows that would be returned were there no LIMIT clause
FROM_DAYS()Convert a day number to a date
FROM_UNIXTIME()Format UNIX timestamp as a date
GeomCollFromText(), GeometryCollectionFromText()Return geometry collection from WKT
GeomCollFromWKB(), GeometryCollectionFromWKB()Return geometry collection from WKB
GeometryCollection()Construct geometry collection from geometries
GeometryN()Return N-th geometry from geometry collection
GeometryType()Return name of geometry type
GeomFromText(), GeometryFromText()Return geometry from WKT
GeomFromWKB()Return geometry from WKB
GET_FORMAT()Return a date format string
GET_LOCK()Get a named lock
GLength()Return length of LineString
>=Greater than or equal operator
>Greater than operator
GREATEST()Return the largest argument
GROUP_CONCAT()Return a concatenated string
HEX()Return a hexadecimal representation of a decimal or string value
HOUR()Extract the hour
IF()If/else construct
IFNULL()Null if/else construct
IN()Check whether a value is within a set of values
INET_ATON()Return the numeric value of an IP address
INET_NTOA()Return the IP address from a numeric value
INSERT()Insert a substring at the specified position up to the specified number of characters
INSTR()Return the index of the first occurrence of substring
InteriorRingN()Return N-th interior ring of Polygon
Intersects()Whether one geometry intersects another
INTERVAL()Return the index of the argument that is less than the first argument
IS_FREE_LOCK()Checks whether the named lock is free
IS NOT NULLNOT NULL value test
IS NOTTest a value against a boolean
IS NULLNULL value test
IS_USED_LOCK()Checks whether the named lock is in use. Return connection identifier if true.
ISTest a value against a boolean
IsClosed()Whether a geometry is closed and simple
IsEmpty()Placeholder function
ISNULL()Test whether the argument is NULL
IsSimple()Whether a geometry is simple
LAST_DAYReturn the last day of the month for the argument
LAST_INSERT_ID()Value of the AUTOINCREMENT column for the last INSERT
LCASE()Synonym for LOWER()
LEAST()Return the smallest argument
<<Left shift
LEFT()Return the leftmost number of characters as specified
LENGTH()Return the length of a string in bytes
<=Less than or equal operator
<Less than operator
LIKESimple pattern matching
LineFromText()Construct LineString from WKT
LineFromWKB(), LineStringFromWKB()Construct LineString from WKB
LineString()Construct LineString from Point values
LN()Return the natural logarithm of the argument
LOAD_FILE()Load the named file
LOCALTIME(), LOCALTIMESynonym for NOW()
LOCALTIMESTAMP, LOCALTIMESTAMP()Synonym for NOW()
LOCATE()Return the position of the first occurrence of substring
LOG10()Return the base-10 logarithm of the argument
LOG2()Return the base-2 logarithm of the argument
LOG()Return the natural logarithm of the first argument
LOWER()Return the argument in lowercase
LPAD()Return the string argument, left-padded with the specified string
LTRIM()Remove leading spaces
MAKE_SET()Return a set of comma-separated strings that have the corresponding bit in bits set
MAKEDATE()Create a date from the year and day of year
MAKETIME()Create time from hour, minute, second
MASTER_POS_WAIT()Block until the slave has read and applied all updates up to the specified position
MATCHPerform full-text search
MAX()Return the maximum value
MBRContains()Whether MBR of one geometry contains MBR of another
MBRDisjoint()Whether MBRs of two geometries are disjoint
MBREqual() (deprecated 5.7.6)Whether MBRs of two geometries are equal
MBRIntersects()Whether MBRs of two geometries intersect
MBROverlaps()Whether MBRs of two geometries overlap
MBRTouches()Whether MBRs of two geometries touch
MBRWithin()Whether MBR of one geometry is within MBR of another
MD5()Calculate MD5 checksum
MICROSECOND()Return the microseconds from argument
MID()Return a substring starting from the specified position
MIN()Return the minimum value
-Minus operator
MINUTE()Return the minute from the argument
MLineFromText(), MultiLineStringFromText()Construct MultiLineString from WKT
MLineFromWKB(), MultiLineStringFromWKB()Construct MultiLineString from WKB
MOD()Return the remainder
% or MODModulo operator
MONTH()Return the month from the date passed
MONTHNAME()Return the name of the month
MPointFromText(), MultiPointFromText()Construct MultiPoint from WKT
MPointFromWKB(), MultiPointFromWKB()Construct MultiPoint from WKB
MPolyFromText(), MultiPolygonFromText()Construct MultiPolygon from WKT
MPolyFromWKB(), MultiPolygonFromWKB()Construct MultiPolygon from WKB
MultiLineString()Contruct MultiLineString from LineString values
MultiPoint()Construct MultiPoint from Point values
MultiPolygon()Construct MultiPolygon from Polygon values
NAME_CONST()Causes the column to have the given name
NOT BETWEEN ... AND ...Check whether a value is not within a range of values
!=, <>Not equal operator
NOT IN()Check whether a value is not within a set of values
NOT LIKENegation of simple pattern matching
NOT REGEXPNegation of REGEXP
NOT, !Negates value
NOW()Return the current date and time
NULLIF()Return NULL if expr1 = expr2
NumGeometries()Return number of geometries in geometry collection
NumInteriorRings()Return number of interior rings in Polygon
NumPoints()Return number of points in LineString
OCT()Return a string containing octal representation of a number
OCTET_LENGTH()Synonym for LENGTH()
OLD_PASSWORD() (deprecated 5.6.5)Return the value of the pre-4.1 implementation of PASSWORD
||, ORLogical OR
ORD()Return character code for leftmost character of the argument
Overlaps()Whether one geometry overlaps another
PASSWORD()Calculate and return a password string
PERIOD_ADD()Add a period to a year-month
PERIOD_DIFF()Return the number of months between periods
PI()Return the value of pi
+Addition operator
Point()Construct Point from coordinates
PointFromText()Construct Point from WKT
PointFromWKB()Construct Point from WKB
PointN()Return N-th point from LineString
PolyFromText(), PolygonFromText()Construct Polygon from WKT
PolyFromWKB(), PolygonFromWKB()Construct Polygon from WKB
Polygon()Construct Polygon from LineString arguments
POSITION()Synonym for LOCATE()
POW()Return the argument raised to the specified power
POWER()Return the argument raised to the specified power
PROCEDURE ANALYSE()Analyze the results of a query
QUARTER()Return the quarter from a date argument
QUOTE()Escape the argument for use in an SQL statement
RADIANS()Return argument converted to radians
RAND()Return a random floating-point value
REGEXPPattern matching using regular expressions
RELEASE_LOCK()Releases the named lock
REPEAT()Repeat a string the specified number of times
REPLACE()Replace occurrences of a specified string
REVERSE()Reverse the characters in a string
>>Right shift
RIGHT()Return the specified rightmost number of characters
RLIKESynonym for REGEXP
ROUND()Round the argument
ROW_COUNT()The number of rows updated
RPAD()Append string the specified number of times
RTRIM()Remove trailing spaces
SCHEMA()Synonym for DATABASE()
SEC_TO_TIME()Converts seconds to 'HH:MM:SS' format
SECOND()Return the second (0-59)
SESSION_USER()Synonym for USER()
SHA1(), SHA()Calculate an SHA-1 160-bit checksum
SHA2()Calculate an SHA-2 checksum
SIGN()Return the sign of the argument
SIN()Return the sine of the argument
SLEEP()Sleep for a number of seconds
SOUNDEX()Return a soundex string
SOUNDS LIKECompare sounds
SPACE()Return a string of the specified number of spaces
SQRT()Return the square root of the argument
SRID()Return spatial reference system ID for geometry
StartPoint()Start Point of LineString
STD()Return the population standard deviation
STDDEV_POP()Return the population standard deviation
STDDEV_SAMP()Return the sample standard deviation
STDDEV()Return the population standard deviation
STR_TO_DATE()Convert a string to a date
STRCMP()Compare two strings
SUBDATE()Synonym for DATE_SUB() when invoked with three arguments
SUBSTR()Return the substring as specified
SUBSTRING_INDEX()Return a substring from a string before the specified number of occurrences of the delimiter
SUBSTRING()Return the substring as specified
SUBTIME()Subtract times
SUM()Return the sum
SYSDATE()Return the time at which the function executes
SYSTEM_USER()Synonym for USER()
TAN()Return the tangent of the argument
TIME_FORMAT()Format as time
TIME_TO_SEC()Return the argument converted to seconds
TIME()Extract the time portion of the expression passed
TIMEDIFF()Subtract time
*Multiplication operator
TIMESTAMP()With a single argument, this function returns the date or datetime expression; with two arguments, the sum of the arguments
TIMESTAMPADD()Add an interval to a datetime expression
TIMESTAMPDIFF()Subtract an interval from a datetime expression
TO_DAYS()Return the date argument converted to days
TO_SECONDS()Return the date or datetime argument converted to seconds since Year 0
Touches()Whether one geometry touches another
TRIM()Remove leading and trailing spaces
TRUNCATE()Truncate to specified number of decimal places
UCASE()Synonym for UPPER()
-Change the sign of the argument
UNCOMPRESS()Uncompress a string compressed
UNCOMPRESSED_LENGTH()Return the length of a string before compression
UNHEX()Return a string containing hex representation of a number
UNIX_TIMESTAMP()Return a UNIX timestamp
UpdateXML()Return replaced XML fragment
UPPER()Convert to uppercase
USER()The user name and host name provided by the client
UTC_DATE()Return the current UTC date
UTC_TIME()Return the current UTC time
UTC_TIMESTAMP()Return the current UTC date and time
UUID_SHORT()Return an integer-valued universal identifier
UUID()Return a Universal Unique Identifier (UUID)
VALUES()Defines the values to be used during an INSERT
VAR_POP()Return the population standard variance
VAR_SAMP()Return the sample variance
VARIANCE()Return the population standard variance
VERSION()Return a string that indicates the MySQL server version
WEEK()Return the week number
WEEKDAY()Return the weekday index
WEEKOFYEAR()Return the calendar week of the date (0-53)
Within()Whether one geometry is within another
X()Return X coordinate of Point
XORLogical XOR
Y()Return Y coordinate of Point
YEAR()Return the year
YEARWEEK()Return the year and week

12.2 Type Conversion in Expression Evaluation

When an operator is used with operands of different types, type conversion occurs to make the operands compatible. Some conversions occur implicitly. For example, MySQL automatically converts numbers to strings as necessary, and vice versa.

mysql> SELECT 1+'1';
        -> 2
mysql> SELECT CONCAT(2,' test');
        -> '2 test'

It is also possible to convert a number to a string explicitly using the CAST() function. Conversion occurs implicitly with the CONCAT() function because it expects string arguments.

mysql> SELECT 38.8, CAST(38.8 AS CHAR);
        -> 38.8, '38.8'
mysql> SELECT 38.8, CONCAT(38.8);
        -> 38.8, '38.8'

See later in this section for information about the character set of implicit number-to-string conversions, and for modified rules that apply to CREATE TABLE ... SELECT statements.

The following rules describe how conversion occurs for comparison operations:

  • If one or both arguments are NULL, the result of the comparison is NULL, except for the NULL-safe <=> equality comparison operator. For NULL <=> NULL, the result is true. No conversion is needed.

  • If both arguments in a comparison operation are strings, they are compared as strings.

  • If both arguments are integers, they are compared as integers.

  • Hexadecimal values are treated as binary strings if not compared to a number.

  • If one of the arguments is a TIMESTAMP or DATETIME column and the other argument is a constant, the constant is converted to a timestamp before the comparison is performed. This is done to be more ODBC-friendly. Note that this is not done for the arguments to IN()! To be safe, always use complete datetime, date, or time strings when doing comparisons. For example, to achieve best results when using BETWEEN with date or time values, use CAST() to explicitly convert the values to the desired data type.

  • If one of the arguments is a decimal value, comparison depends on the other argument. The arguments are compared as decimal values if the other argument is a decimal or integer value, or as floating-point values if the other argument is a floating-point value.

  • In all other cases, the arguments are compared as floating-point (real) numbers.

For information about conversion of values from one temporal type to another, see Section 11.3.7, “Conversion Between Date and Time Types”.

The following examples illustrate conversion of strings to numbers for comparison operations:

mysql> SELECT 1 > '6x';
        -> 0
mysql> SELECT 7 > '6x';
        -> 1
mysql> SELECT 0 > 'x6';
        -> 0
mysql> SELECT 0 = 'x6';
        -> 1

For comparisons of a string column with a number, MySQL cannot use an index on the column to look up the value quickly. If str_col is an indexed string column, the index cannot be used when performing the lookup in the following statement:

SELECT * FROM tbl_name WHERE str_col=1;

The reason for this is that there are many different strings that may convert to the value 1, such as '1', ' 1', or '1a'.

Comparisons that use floating-point numbers (or values that are converted to floating-point numbers) are approximate because such numbers are inexact. This might lead to results that appear inconsistent:

mysql> SELECT '18015376320243458' = 18015376320243458;
        -> 1
mysql> SELECT '18015376320243459' = 18015376320243459;
        -> 0

Such results can occur because the values are converted to floating-point numbers, which have only 53 bits of precision and are subject to rounding:

mysql> SELECT '18015376320243459'+0.0;
        -> 1.8015376320243e+16

Furthermore, the conversion from string to floating-point and from integer to floating-point do not necessarily occur the same way. The integer may be converted to floating-point by the CPU, whereas the string is converted digit by digit in an operation that involves floating-point multiplications.

The results shown will vary on different systems, and can be affected by factors such as computer architecture or the compiler version or optimization level. One way to avoid such problems is to use CAST() so that a value is not converted implicitly to a float-point number:

mysql> SELECT CAST('18015376320243459' AS UNSIGNED) = 18015376320243459;
        -> 1

For more information about floating-point comparisons, see Section B.5.5.8, “Problems with Floating-Point Values”.

As of MySQL 5.5.3, the server includes dtoa, a conversion library that provides the basis for improved conversion between string or DECIMAL values and approximate-value (FLOAT/DOUBLE) numbers:

  • Consistent conversion results across platforms, which eliminates, for example, Unix versus Windows conversion differences.

  • Accurate representation of values in cases where results previously did not provide sufficient precision, such as for values close to IEEE limits.

  • Conversion of numbers to string format with the best possible precision. The precision of dtoa is always the same or better than that of the standard C library functions.

Because the conversions produced by this library differ in some cases from non-dtoa results, the potential exists for incompatibilities in applications that rely on previous results. For example, applications that depend on a specific exact result from previous conversions might need adjustment to accommodate additional precision.

The dtoa library provides conversions with the following properties. D represents a value with a DECIMAL or string representation, and F represents a floating-point number in native binary (IEEE) format.

  • F -> D conversion is done with the best possible precision, returning D as the shortest string that yields F when read back in and rounded to the nearest value in native binary format as specified by IEEE.

  • D -> F conversion is done such that F is the nearest native binary number to the input decimal string D.

These properties imply that F -> D -> F conversions are lossless unless F is -inf, +inf, or NaN. The latter values are not supported because the SQL standard defines them as invalid values for FLOAT or DOUBLE.

For D -> F -> D conversions, a sufficient condition for losslessness is that D uses 15 or fewer digits of precision, is not a denormal value, -inf, +inf, or NaN. In some cases, the conversion is lossless even if D has more than 15 digits of precision, but this is not always the case.

As of MySQL 5.5.3, implicit conversion of a numeric or temporal value to string produces a value that has a character set and collation determined by the character_set_connection and collation_connection system variables. (These variables commonly are set with SET NAMES. For information about connection character sets, see Section 10.1.4, “Connection Character Sets and Collations”.)

This change means that such a conversion results in a character (nonbinary) string (a CHAR, VARCHAR, or LONGTEXT value), except when the connection character set is set to binary. In that case, the conversion result is a binary string (a BINARY, VARBINARY, or LONGBLOB value).

Before MySQL 5.5.3, an implicit conversion always produced a binary string, regardless of the connection character set. Such implicit conversions to string typically occur for functions that are passed numeric or temporal values when string values are more usual, and thus could have effects beyond the type of the converted value. Consider the expression CONCAT(1, 'abc'). The numeric argument 1 was converted to the binary string '1' and the concatenation of that value with the nonbinary string 'abc' produced the binary string '1abc'.

Some functions are unaffected by this change in behavior:

  • CHAR() without a USING clause still returns VARBINARY.

  • Functions that previously returned utf8 strings still do so. Examples include CHARSET() and COLLATION().

  • Encryption and compression functions that expect string arguments and previously returned binary strings are unaffected if the return value can contain non-ASCII characters. Examples include AES_ENCRYPT() and COMPRESS(). If the return value contains only ASCII characters, the function now returns a character string with the connection character set and collation. Examples include MD5() and PASSWORD().

12.3 Operators

Table 12.2 Operators

NameDescription
AND, &&Logical AND
=Assign a value (as part of a SET statement, or as part of the SET clause in an UPDATE statement)
:=Assign a value
BETWEEN ... AND ...Check whether a value is within a range of values
BINARYCast a string to a binary string
&Bitwise AND
~Invert bits
|Bitwise OR
^Bitwise XOR
CASECase operator
DIVInteger division
/Division operator
<=>NULL-safe equal to operator
=Equal operator
>=Greater than or equal operator
>Greater than operator
IS NOT NULLNOT NULL value test
IS NOTTest a value against a boolean
IS NULLNULL value test
ISTest a value against a boolean
<<Left shift
<=Less than or equal operator
<Less than operator
LIKESimple pattern matching
-Minus operator
% or MODModulo operator
NOT BETWEEN ... AND ...Check whether a value is not within a range of values
!=, <>Not equal operator
NOT LIKENegation of simple pattern matching
NOT REGEXPNegation of REGEXP
NOT, !Negates value
||, ORLogical OR
+Addition operator
REGEXPPattern matching using regular expressions
>>Right shift
RLIKESynonym for REGEXP
SOUNDS LIKECompare sounds
*Multiplication operator
-Change the sign of the argument
XORLogical XOR

12.3.1 Operator Precedence

Operator precedences are shown in the following list, from highest precedence to the lowest. Operators that are shown together on a line have the same precedence.

INTERVAL
BINARY, COLLATE
!
- (unary minus), ~ (unary bit inversion)
^
*, /, DIV, %, MOD
-, +
<<, >>
&
|
= (comparison), <=>, >=, >, <=, <, <>, !=, IS, LIKE, REGEXP, IN
BETWEEN, CASE, WHEN, THEN, ELSE
NOT
&&, AND
XOR
||, OR
= (assignment), :=

The precedence of = depends on whether it is used as a comparison operator (=) or as an assignment operator (=). When used as a comparison operator, it has the same precedence as <=>, >=, >, <=, <, <>, !=, IS, LIKE, REGEXP, and IN. When used as an assignment operator, it has the same precedence as :=. Section 13.7.4, “SET Syntax”, and Section 9.4, “User-Defined Variables”, explain how MySQL determines which interpretation of = should apply.

The meaning of some operators depends on the SQL mode:

  • By default, || is a logical OR operator. With PIPES_AS_CONCAT enabled, || is string concatenation, with a precedence between ^ and the unary operators.

  • By default, ! has a higher precedence than NOT. With HIGH_NOT_PRECEDENCE enabled, ! and NOT have the same precedence.

See Section 5.1.7, “Server SQL Modes”.

The precedence of operators determines the order of evaluation of terms in an expression. To override this order and group terms explicitly, use parentheses. For example:

mysql> SELECT 1+2*3;
        -> 7
mysql> SELECT (1+2)*3;
        -> 9

12.3.2 Comparison Functions and Operators

Table 12.3 Comparison Operators

NameDescription
BETWEEN ... AND ...Check whether a value is within a range of values
COALESCE()Return the first non-NULL argument
<=>NULL-safe equal to operator
=Equal operator
>=Greater than or equal operator
>Greater than operator
GREATEST()Return the largest argument
IN()Check whether a value is within a set of values
INTERVAL()Return the index of the argument that is less than the first argument
IS NOT NULLNOT NULL value test
IS NOTTest a value against a boolean
IS NULLNULL value test
ISTest a value against a boolean
ISNULL()Test whether the argument is NULL
LEAST()Return the smallest argument
<=Less than or equal operator
<Less than operator
LIKESimple pattern matching
NOT BETWEEN ... AND ...Check whether a value is not within a range of values
!=, <>Not equal operator
NOT IN()Check whether a value is not within a set of values
NOT LIKENegation of simple pattern matching
STRCMP()Compare two strings

Comparison operations result in a value of 1 (TRUE), 0 (FALSE), or NULL. These operations work for both numbers and strings. Strings are automatically converted to numbers and numbers to strings as necessary.

The following relational comparison operators can be used to compare not only scalar operands, but row operands:

=  >  <  >=  <=  <>  !=

For examples of row comparisons, see Section 13.2.10.5, “Row Subqueries”.

Some of the functions in this section return values other than 1 (TRUE), 0 (FALSE), or NULL. For example, LEAST() and GREATEST(). However, the value they return is based on comparison operations performed according to the rules described in Section 12.2, “Type Conversion in Expression Evaluation”.

To convert a value to a specific type for comparison purposes, you can use the CAST() function. String values can be converted to a different character set using CONVERT(). See Section 12.10, “Cast Functions and Operators”.

By default, string comparisons are not case sensitive and use the current character set. The default is latin1 (cp1252 West European), which also works well for English.

  • =

    Equal:

    mysql> SELECT 1 = 0;
            -> 0
    mysql> SELECT '0' = 0;
            -> 1
    mysql> SELECT '0.0' = 0;
            -> 1
    mysql> SELECT '0.01' = 0;
            -> 0
    mysql> SELECT '.01' = 0.01;
            -> 1
    
  • <=>

    NULL-safe equal. This operator performs an equality comparison like the = operator, but returns 1 rather than NULL if both operands are NULL, and 0 rather than NULL if one operand is NULL.

    mysql> SELECT 1 <=> 1, NULL <=> NULL, 1 <=> NULL;
            -> 1, 1, 0
    mysql> SELECT 1 = 1, NULL = NULL, 1 = NULL;
            -> 1, NULL, NULL
    
  • <>, !=

    Not equal:

    mysql> SELECT '.01' <> '0.01';
            -> 1
    mysql> SELECT .01 <> '0.01';
            -> 0
    mysql> SELECT 'zapp' <> 'zappp';
            -> 1
    
  • <=

    Less than or equal:

    mysql> SELECT 0.1 <= 2;
            -> 1
    
  • <

    Less than:

    mysql> SELECT 2 < 2;
            -> 0
    
  • >=

    Greater than or equal:

    mysql> SELECT 2 >= 2;
            -> 1
    
  • >

    Greater than:

    mysql> SELECT 2 > 2;
            -> 0
    
  • IS boolean_value

    Tests a value against a boolean value, where boolean_value can be TRUE, FALSE, or UNKNOWN.

    mysql> SELECT 1 IS TRUE, 0 IS FALSE, NULL IS UNKNOWN;
            -> 1, 1, 1
    
  • IS NOT boolean_value

    Tests a value against a boolean value, where boolean_value can be TRUE, FALSE, or UNKNOWN.

    mysql> SELECT 1 IS NOT UNKNOWN, 0 IS NOT UNKNOWN, NULL IS NOT UNKNOWN;
            -> 1, 1, 0
    
  • IS NULL

    Tests whether a value is NULL.

    mysql> SELECT 1 IS NULL, 0 IS NULL, NULL IS NULL;
            -> 0, 0, 1
    

    To work well with ODBC programs, MySQL supports the following extra features when using IS NULL:

    • If sql_auto_is_null variable is set to 1, then after a statement that successfully inserts an automatically generated AUTO_INCREMENT value, you can find that value by issuing a statement of the following form:

      SELECT * FROM tbl_name WHERE auto_col IS NULL
      

      If the statement returns a row, the value returned is the same as if you invoked the LAST_INSERT_ID() function. For details, including the return value after a multiple-row insert, see Section 12.14, “Information Functions”. If no AUTO_INCREMENT value was successfully inserted, the SELECT statement returns no row.

      The behavior of retrieving an AUTO_INCREMENT value by using an IS NULL comparison can be disabled by setting sql_auto_is_null = 0. See Section 5.1.4, “Server System Variables”.

      The default value of sql_auto_is_null is 0 as of MySQL 5.5.3, and 1 for earlier versions.

    • For DATE and DATETIME columns that are declared as NOT NULL, you can find the special date '0000-00-00' by using a statement like this:

      SELECT * FROM tbl_name WHERE date_column IS NULL
      

      This is needed to get some ODBC applications to work because ODBC does not support a '0000-00-00' date value.

      See Obtaining Auto-Increment Values, and the description for the FLAG_AUTO_IS_NULL option at Connector/ODBC Connection Parameters.

  • IS NOT NULL

    Tests whether a value is not NULL.

    mysql> SELECT 1 IS NOT NULL, 0 IS NOT NULL, NULL IS NOT NULL;
            -> 1, 1, 0
    
  • expr BETWEEN min AND max

    If expr is greater than or equal to min and expr is less than or equal to max, BETWEEN returns 1, otherwise it returns 0. This is equivalent to the expression (min <= expr AND expr <= max) if all the arguments are of the same type. Otherwise type conversion takes place according to the rules described in Section 12.2, “Type Conversion in Expression Evaluation”, but applied to all the three arguments.

    mysql> SELECT 2 BETWEEN 1 AND 3, 2 BETWEEN 3 and 1;
            -> 1, 0
    mysql> SELECT 1 BETWEEN 2 AND 3;
            -> 0
    mysql> SELECT 'b' BETWEEN 'a' AND 'c';
            -> 1
    mysql> SELECT 2 BETWEEN 2 AND '3';
            -> 1
    mysql> SELECT 2 BETWEEN 2 AND 'x-3';
            -> 0
    

    For best results when using BETWEEN with date or time values, use CAST() to explicitly convert the values to the desired data type. Examples: If you compare a DATETIME to two DATE values, convert the DATE values to DATETIME values. If you use a string constant such as '2001-1-1' in a comparison to a DATE, cast the string to a DATE.

  • expr NOT BETWEEN min AND max

    This is the same as NOT (expr BETWEEN min AND max).

  • COALESCE(value,...)

    Returns the first non-NULL value in the list, or NULL if there are no non-NULL values.

    mysql> SELECT COALESCE(NULL,1);
            -> 1
    mysql> SELECT COALESCE(NULL,NULL,NULL);
            -> NULL
    
  • GREATEST(value1,value2,...)

    With two or more arguments, returns the largest (maximum-valued) argument. The arguments are compared using the same rules as for LEAST().

    mysql> SELECT GREATEST(2,0);
            -> 2
    mysql> SELECT GREATEST(34.0,3.0,5.0,767.0);
            -> 767.0
    mysql> SELECT GREATEST('B','A','C');
            -> 'C'
    

    GREATEST() returns NULL if any argument is NULL.

  • expr IN (value,...)

    Returns 1 if expr is equal to any of the values in the IN list, else returns 0. If all values are constants, they are evaluated according to the type of expr and sorted. The search for the item then is done using a binary search. This means IN is very quick if the IN value list consists entirely of constants. Otherwise, type conversion takes place according to the rules described in Section 12.2, “Type Conversion in Expression Evaluation”, but applied to all the arguments.

    mysql> SELECT 2 IN (0,3,5,7);
            -> 0
    mysql> SELECT 'wefwf' IN ('wee','wefwf','weg');
            -> 1
    

    You should never mix quoted and unquoted values in an IN list because the comparison rules for quoted values (such as strings) and unquoted values (such as numbers) differ. Mixing types may therefore lead to inconsistent results. For example, do not write an IN expression like this:

    SELECT val1 FROM tbl1 WHERE val1 IN (1,2,'a');
    

    Instead, write it like this:

    SELECT val1 FROM tbl1 WHERE val1 IN ('1','2','a');
    

    The number of values in the IN list is only limited by the max_allowed_packet value.

    To comply with the SQL standard, IN returns NULL not only if the expression on the left hand side is NULL, but also if no match is found in the list and one of the expressions in the list is NULL.

    IN() syntax can also be used to write certain types of subqueries. See Section 13.2.10.3, “Subqueries with ANY, IN, or SOME”.

  • expr NOT IN (value,...)

    This is the same as NOT (expr IN (value,...)).

  • ISNULL(expr)

    If expr is NULL, ISNULL() returns 1, otherwise it returns 0.

    mysql> SELECT ISNULL(1+1);
            -> 0
    mysql> SELECT ISNULL(1/0);
            -> 1
    

    ISNULL() can be used instead of = to test whether a value is NULL. (Comparing a value to NULL using = always yields false.)

    The ISNULL() function shares some special behaviors with the IS NULL comparison operator. See the description of IS NULL.

  • INTERVAL(N,N1,N2,N3,...)

    Returns 0 if N < N1, 1 if N < N2 and so on or -1 if N is NULL. All arguments are treated as integers. It is required that N1 < N2 < N3 < ... < Nn for this function to work correctly. This is because a binary search is used (very fast).

    mysql> SELECT INTERVAL(23, 1, 15, 17, 30, 44, 200);
            -> 3
    mysql> SELECT INTERVAL(10, 1, 10, 100, 1000);
            -> 2
    mysql> SELECT INTERVAL(22, 23, 30, 44, 200);
            -> 0
    
  • LEAST(value1,value2,...)

    With two or more arguments, returns the smallest (minimum-valued) argument. The arguments are compared using the following rules:

    • If any argument is NULL, the result is NULL. No comparison is needed.

    • If the return value is used in an INTEGER context or all arguments are integer-valued, they are compared as integers.

    • If the return value is used in a REAL context or all arguments are real-valued, they are compared as reals.

    • If the arguments comprise a mix of numbers and strings, they are compared as numbers.

    • If any argument is a nonbinary (character) string, the arguments are compared as nonbinary strings.

    • In all other cases, the arguments are compared as binary strings.

    mysql> SELECT LEAST(2,0);
            -> 0
    mysql> SELECT LEAST(34.0,3.0,5.0,767.0);
            -> 3.0
    mysql> SELECT LEAST('B','A','C');
            -> 'A'
    

    Note that the preceding conversion rules can produce strange results in some borderline cases:

    mysql> SELECT CAST(LEAST(3600, 9223372036854775808.0) as SIGNED);
            -> -9223372036854775808
    

    This happens because MySQL reads 9223372036854775808.0 in an integer context. The integer representation is not good enough to hold the value, so it wraps to a signed integer.

12.3.3 Logical Operators

Table 12.4 Logical Operators

NameDescription
AND, &&Logical AND
NOT, !Negates value
||, ORLogical OR
XORLogical XOR

In SQL, all logical operators evaluate to TRUE, FALSE, or NULL (UNKNOWN). In MySQL, these are implemented as 1 (TRUE), 0 (FALSE), and NULL. Most of this is common to different SQL database servers, although some servers may return any nonzero value for TRUE.

MySQL evaluates any nonzero, non-NULL value to TRUE. For example, the following statements all assess to TRUE:

mysql> SELECT 10 IS TRUE;
-> 1
mysql> SELECT -10 IS TRUE;
-> 1
mysql> SELECT 'string' IS NOT NULL;
-> 1
  • NOT, !

    Logical NOT. Evaluates to 1 if the operand is 0, to 0 if the operand is nonzero, and NOT NULL returns NULL.

    mysql> SELECT NOT 10;
            -> 0
    mysql> SELECT NOT 0;
            -> 1
    mysql> SELECT NOT NULL;
            -> NULL
    mysql> SELECT ! (1+1);
            -> 0
    mysql> SELECT ! 1+1;
            -> 1
    

    The last example produces 1 because the expression evaluates the same way as (!1)+1.

  • AND, &&

    Logical AND. Evaluates to 1 if all operands are nonzero and not NULL, to 0 if one or more operands are 0, otherwise NULL is returned.

    mysql> SELECT 1 && 1;
            -> 1
    mysql> SELECT 1 && 0;
            -> 0
    mysql> SELECT 1 && NULL;
            -> NULL
    mysql> SELECT 0 && NULL;
            -> 0
    mysql> SELECT NULL && 0;
            -> 0
    
  • OR, ||

    Logical OR. When both operands are non-NULL, the result is 1 if any operand is nonzero, and 0 otherwise. With a NULL operand, the result is 1 if the other operand is nonzero, and NULL otherwise. If both operands are NULL, the result is NULL.

    mysql> SELECT 1 || 1;
            -> 1
    mysql> SELECT 1 || 0;
            -> 1
    mysql> SELECT 0 || 0;
            -> 0
    mysql> SELECT 0 || NULL;
            -> NULL
    mysql> SELECT 1 || NULL;
            -> 1
    
  • XOR

    Logical XOR. Returns NULL if either operand is NULL. For non-NULL operands, evaluates to 1 if an odd number of operands is nonzero, otherwise 0 is returned.

    mysql> SELECT 1 XOR 1;
            -> 0
    mysql> SELECT 1 XOR 0;
            -> 1
    mysql> SELECT 1 XOR NULL;
            -> NULL
    mysql> SELECT 1 XOR 1 XOR 1;
            -> 1
    

    a XOR b is mathematically equal to (a AND (NOT b)) OR ((NOT a) and b).

12.3.4 Assignment Operators

Table 12.5 Assignment Operators

NameDescription
=Assign a value (as part of a SET statement, or as part of the SET clause in an UPDATE statement)
:=Assign a value

  • :=

    Assignment operator. Causes the user variable on the left hand side of the operator to take on the value to its right. The value on the right hand side may be a literal value, another variable storing a value, or any legal expression that yields a scalar value, including the result of a query (provided that this value is a scalar value). You can perform multiple assignments in the same SET statement. You can perform multiple assignments in the same statement-

    Unlike =, the := operator is never interpreted as a comparison operator. This means you can use := in any valid SQL statement (not just in SET statements) to assign a value to a variable.

    mysql> SELECT @var1, @var2;
            -> NULL, NULL
    mysql> SELECT @var1 := 1, @var2;
            -> 1, NULL
    mysql> SELECT @var1, @var2;
            -> 1, NULL
    mysql> SELECT @var1, @var2 := @var1;
            -> 1, 1
    mysql> SELECT @var1, @var2;
            -> 1, 1
    
    mysql> SELECT @var1:=COUNT(*) FROM t1;
            -> 4
    mysql> SELECT @var1;
            -> 4
    

    You can make value assignments using := in other statements besides SELECT, such as UPDATE, as shown here:

    mysql> SELECT @var1;
            -> 4
    mysql> SELECT * FROM t1;
            -> 1, 3, 5, 7
    
    mysql> UPDATE t1 SET c1 = 2 WHERE c1 = @var1:= 1;
    Query OK, 1 row affected (0.00 sec)
    Rows matched: 1  Changed: 1  Warnings: 0
    
    mysql> SELECT @var1;
            -> 1
    mysql> SELECT * FROM t1;
            -> 2, 3, 5, 7
    

    While it is also possible both to set and to read the value of the same variable in a single SQL statement using the := operator, this is not recommended. Section 9.4, “User-Defined Variables”, explains why you should avoid doing this.

  • =

    This operator is used to perform value assignments in two cases, described in the next two paragraphs.

    Within a SET statement, = is treated as an assignment operator that causes the user variable on the left hand side of the operator to take on the value to its right. (In other words, when used in a SET statement, = is treated identically to :=.) The value on the right hand side may be a literal value, another variable storing a value, or any legal expression that yields a scalar value, including the result of a query (provided that this value is a scalar value). You can perform multiple assignments in the same SET statement.

    In the SET clause of an UPDATE statement, = also acts as an assignment operator; in this case, however, it causes the column named on the left hand side of the operator to assume the value given to the right, provided any WHERE conditions that are part of the UPDATE are met. You can make multiple assignments in the same SET clause of an UPDATE statement.

    In any other context, = is treated as a comparison operator.

    mysql> SELECT @var1, @var2;
            -> NULL, NULL
    mysql> SELECT @var1 := 1, @var2;
            -> 1, NULL
    mysql> SELECT @var1, @var2;
            -> 1, NULL
    mysql> SELECT @var1, @var2 := @var1;
            -> 1, 1
    mysql> SELECT @var1, @var2;
            -> 1, 1
    

    For more information, see Section 13.7.4, “SET Syntax”, Section 13.2.11, “UPDATE Syntax”, and Section 13.2.10, “Subquery Syntax”.

12.4 Control Flow Functions

Table 12.6 Flow Control Operators

NameDescription
CASECase operator
IF()If/else construct
IFNULL()Null if/else construct
NULLIF()Return NULL if expr1 = expr2

  • CASE value WHEN [compare_value] THEN result [WHEN [compare_value] THEN result ...] [ELSE result] END

    CASE WHEN [condition] THEN result [WHEN [condition] THEN result ...] [ELSE result] END

    The first version returns the result where value=compare_value. The second version returns the result for the first condition that is true. If there was no matching result value, the result after ELSE is returned, or NULL if there is no ELSE part.

    mysql> SELECT CASE 1 WHEN 1 THEN 'one'
        ->     WHEN 2 THEN 'two' ELSE 'more' END;
            -> 'one'
    mysql> SELECT CASE WHEN 1>0 THEN 'true' ELSE 'false' END;
            -> 'true'
    mysql> SELECT CASE BINARY 'B'
        ->     WHEN 'a' THEN 1 WHEN 'b' THEN 2 END;
            -> NULL
    

    The return type of a CASE expression is the compatible aggregated type of all return values, but also depends on the context in which it is used. If used in a string context, the result is returned as a string. If used in a numeric context, the result is returned as a decimal, real, or integer value.

    Note

    The syntax of the CASE expression shown here differs slightly from that of the SQL CASE statement described in Section 13.6.5.1, “CASE Syntax”, for use inside stored programs. The CASE statement cannot have an ELSE NULL clause, and it is terminated with END CASE instead of END.

  • IF(expr1,expr2,expr3)

    If expr1 is TRUE (expr1 <> 0 and expr1 <> NULL) then IF() returns expr2; otherwise it returns expr3. IF() returns a numeric or string value, depending on the context in which it is used.

    mysql> SELECT IF(1>2,2,3);
            -> 3
    mysql> SELECT IF(1<2,'yes','no');
            -> 'yes'
    mysql> SELECT IF(STRCMP('test','test1'),'no','yes');
            -> 'no'
    

    If only one of expr2 or expr3 is explicitly NULL, the result type of the IF() function is the type of the non-NULL expression.

    The default return type of IF() (which may matter when it is stored into a temporary table) is calculated as follows.

    ExpressionReturn Value
    expr2 or expr3 returns a stringstring
    expr2 or expr3 returns a floating-point valuefloating-point
    expr2 or expr3 returns an integerinteger

    If expr2 and expr3 are both strings, the result is case sensitive if either string is case sensitive.

    Note

    There is also an IF statement, which differs from the IF() function described here. See Section 13.6.5.2, “IF Syntax”.

  • IFNULL(expr1,expr2)

    If expr1 is not NULL, IFNULL() returns expr1; otherwise it returns expr2. IFNULL() returns a numeric or string value, depending on the context in which it is used.

    mysql> SELECT IFNULL(1,0);
            -> 1
    mysql> SELECT IFNULL(NULL,10);
            -> 10
    mysql> SELECT IFNULL(1/0,10);
            -> 10
    mysql> SELECT IFNULL(1/0,'yes');
            -> 'yes'
    

    The default result value of IFNULL(expr1,expr2) is the more general of the two expressions, in the order STRING, REAL, or INTEGER. Consider the case of a table based on expressions or where MySQL must internally store a value returned by IFNULL() in a temporary table:

    
    mysql> CREATE TABLE tmp SELECT IFNULL(1,'test') AS test;
    mysql> DESCRIBE tmp;
    +-------+--------------+------+-----+---------+-------+
    | Field | Type         | Null | Key | Default | Extra |
    +-------+--------------+------+-----+---------+-------+
    | test  | varbinary(4) | NO   |     |         |       |
    +-------+--------------+------+-----+---------+-------+
    

    In this example, the type of the test column is VARBINARY(4).

  • NULLIF(expr1,expr2)

    Returns NULL if expr1 = expr2 is true, otherwise returns expr1. This is the same as CASE WHEN expr1 = expr2 THEN NULL ELSE expr1 END.

    mysql> SELECT NULLIF(1,1);
            -> NULL
    mysql> SELECT NULLIF(1,2);
            -> 1
    

    Note that MySQL evaluates expr1 twice if the arguments are not equal.

12.5 String Functions

Table 12.7 String Operators

NameDescription
ASCII()Return numeric value of left-most character
BIN()Return a string containing binary representation of a number
BIT_LENGTH()Return length of argument in bits
CHAR_LENGTH()Return number of characters in argument
CHAR()Return the character for each integer passed
CHARACTER_LENGTH()Synonym for CHAR_LENGTH()
CONCAT_WS()Return concatenate with separator
CONCAT()Return concatenated string
ELT()Return string at index number
EXPORT_SET()Return a string such that for every bit set in the value bits, you get an on string and for every unset bit, you get an off string
FIELD()Return the index (position) of the first argument in the subsequent arguments
FIND_IN_SET()Return the index position of the first argument within the second argument
FORMAT()Return a number formatted to specified number of decimal places
HEX()Return a hexadecimal representation of a decimal or string value
INSERT()Insert a substring at the specified position up to the specified number of characters
INSTR()Return the index of the first occurrence of substring
LCASE()Synonym for LOWER()
LEFT()Return the leftmost number of characters as specified
LENGTH()Return the length of a string in bytes
LIKESimple pattern matching
LOAD_FILE()Load the named file
LOCATE()Return the position of the first occurrence of substring
LOWER()Return the argument in lowercase
LPAD()Return the string argument, left-padded with the specified string
LTRIM()Remove leading spaces
MAKE_SET()Return a set of comma-separated strings that have the corresponding bit in bits set
MATCHPerform full-text search
MID()Return a substring starting from the specified position
NOT LIKENegation of simple pattern matching
NOT REGEXPNegation of REGEXP
OCT()Return a string containing octal representation of a number
OCTET_LENGTH()Synonym for LENGTH()
ORD()Return character code for leftmost character of the argument
POSITION()Synonym for LOCATE()
QUOTE()Escape the argument for use in an SQL statement
REGEXPPattern matching using regular expressions
REPEAT()Repeat a string the specified number of times
REPLACE()Replace occurrences of a specified string
REVERSE()Reverse the characters in a string
RIGHT()Return the specified rightmost number of characters
RLIKESynonym for REGEXP
RPAD()Append string the specified number of times
RTRIM()Remove trailing spaces
SOUNDEX()Return a soundex string
SOUNDS LIKECompare sounds
SPACE()Return a string of the specified number of spaces
STRCMP()Compare two strings
SUBSTR()Return the substring as specified
SUBSTRING_INDEX()Return a substring from a string before the specified number of occurrences of the delimiter
SUBSTRING()Return the substring as specified
TRIM()Remove leading and trailing spaces
UCASE()Synonym for UPPER()
UNHEX()Return a string containing hex representation of a number
UPPER()Convert to uppercase

String-valued functions return NULL if the length of the result would be greater than the value of the max_allowed_packet system variable. See Section 8.11.2, “Tuning Server Parameters”.

For functions that operate on string positions, the first position is numbered 1.

For functions that take length arguments, noninteger arguments are rounded to the nearest integer.

  • ASCII(str)

    Returns the numeric value of the leftmost character of the string str. Returns 0 if str is the empty string. Returns NULL if str is NULL. ASCII() works for 8-bit characters.

    mysql> SELECT ASCII('2');
            -> 50
    mysql> SELECT ASCII(2);
            -> 50
    mysql> SELECT ASCII('dx');
            -> 100
    

    See also the ORD() function.

  • BIN(N)

    Returns a string representation of the binary value of N, where N is a longlong (BIGINT) number. This is equivalent to CONV(N,10,2). Returns NULL if N is NULL.

    mysql> SELECT BIN(12);
            -> '1100'
    
  • BIT_LENGTH(str)

    Returns the length of the string str in bits.

    mysql> SELECT BIT_LENGTH('text');
            -> 32
    
  • CHAR(N,... [USING charset_name])

    CHAR() interprets each argument N as an integer and returns a string consisting of the characters given by the code values of those integers. NULL values are skipped.

    mysql> SELECT CHAR(77,121,83,81,'76');
            -> 'MySQL'
    mysql> SELECT CHAR(77,77.3,'77.3');
            -> 'MMM'
    

    CHAR() arguments larger than 255 are converted into multiple result bytes. For example, CHAR(256) is equivalent to CHAR(1,0), and CHAR(256*256) is equivalent to CHAR(1,0,0):

    mysql> SELECT HEX(CHAR(1,0)), HEX(CHAR(256));
    +----------------+----------------+
    | HEX(CHAR(1,0)) | HEX(CHAR(256)) |
    +----------------+----------------+
    | 0100           | 0100           |
    +----------------+----------------+
    mysql> SELECT HEX(CHAR(1,0,0)), HEX(CHAR(256*256));
    +------------------+--------------------+
    | HEX(CHAR(1,0,0)) | HEX(CHAR(256*256)) |
    +------------------+--------------------+
    | 010000           | 010000             |
    +------------------+--------------------+
    

    By default, CHAR() returns a binary string. To produce a string in a given character set, use the optional USING clause:

    mysql> SELECT CHARSET(CHAR(0x65)), CHARSET(CHAR(0x65 USING utf8));
    +---------------------+--------------------------------+
    | CHARSET(CHAR(0x65)) | CHARSET(CHAR(0x65 USING utf8)) |
    +---------------------+--------------------------------+
    | binary              | utf8                           |
    +---------------------+--------------------------------+
    

    If USING is given and the result string is illegal for the given character set, a warning is issued. Also, if strict SQL mode is enabled, the result from CHAR() becomes NULL.

  • CHAR_LENGTH(str)

    Returns the length of the string str, measured in characters. A multibyte character counts as a single character. This means that for a string containing five 2-byte characters, LENGTH() returns 10, whereas CHAR_LENGTH() returns 5.

  • CHARACTER_LENGTH(str)

    CHARACTER_LENGTH() is a synonym for CHAR_LENGTH().

  • CONCAT(str1,str2,...)

    Returns the string that results from concatenating the arguments. May have one or more arguments. If all arguments are nonbinary strings, the result is a nonbinary string. If the arguments include any binary strings, the result is a binary string. A numeric argument is converted to its equivalent string form. This is a nonbinary string as of MySQL 5.5.3. Before 5.5.3, it is a binary string; to avoid that and produce a nonbinary string, you can use an explicit type cast, as in this example:

    SELECT CONCAT(CAST(int_col AS CHAR), char_col);
    

    CONCAT() returns NULL if any argument is NULL.

    mysql> SELECT CONCAT('My', 'S', 'QL');
            -> 'MySQL'
    mysql> SELECT CONCAT('My', NULL, 'QL');
            -> NULL
    mysql> SELECT CONCAT(14.3);
            -> '14.3'
    

    For quoted strings, concatenation can be performed by placing the strings next to each other:

    mysql> SELECT 'My' 'S' 'QL';
            -> 'MySQL'
    
  • CONCAT_WS(separator,str1,str2,...)

    CONCAT_WS() stands for Concatenate With Separator and is a special form of CONCAT(). The first argument is the separator for the rest of the arguments. The separator is added between the strings to be concatenated. The separator can be a string, as can the rest of the arguments. If the separator is NULL, the result is NULL.

    mysql> SELECT CONCAT_WS(',','First name','Second name','Last Name');
            -> 'First name,Second name,Last Name'
    mysql> SELECT CONCAT_WS(',','First name',NULL,'Last Name');
            -> 'First name,Last Name'
    

    CONCAT_WS() does not skip empty strings. However, it does skip any NULL values after the separator argument.

  • ELT(N,str1,str2,str3,...)

    ELT() returns the Nth element of the list of strings: str1 if N = 1, str2 if N = 2, and so on. Returns NULL if N is less than 1 or greater than the number of arguments. ELT() is the complement of FIELD().

    mysql> SELECT ELT(1, 'ej', 'Heja', 'hej', 'foo');
            -> 'ej'
    mysql> SELECT ELT(4, 'ej', 'Heja', 'hej', 'foo');
            -> 'foo'
    
  • EXPORT_SET(bits,on,off[,separator[,number_of_bits]])

    Returns a string such that for every bit set in the value bits, you get an on string and for every bit not set in the value, you get an off string. Bits in bits are examined from right to left (from low-order to high-order bits). Strings are added to the result from left to right, separated by the separator string (the default being the comma character ,). The number of bits examined is given by number_of_bits, which has a default of 64 if not specified. number_of_bits is silently clipped to 64 if larger than 64. It is treated as an unsigned integer, so a value of –1 is effectively the same as 64.

    mysql> SELECT EXPORT_SET(5,'Y','N',',',4);
            -> 'Y,N,Y,N'
    mysql> SELECT EXPORT_SET(6,'1','0',',',10);
            -> '0,1,1,0,0,0,0,0,0,0'
    
  • FIELD(str,str1,str2,str3,...)

    Returns the index (position) of str in the str1, str2, str3, ... list. Returns 0 if str is not found.

    If all arguments to FIELD() are strings, all arguments are compared as strings. If all arguments are numbers, they are compared as numbers. Otherwise, the arguments are compared as double.

    If str is NULL, the return value is 0 because NULL fails equality comparison with any value. FIELD() is the complement of ELT().

    mysql> SELECT FIELD('ej', 'Hej', 'ej', 'Heja', 'hej', 'foo');
            -> 2
    mysql> SELECT FIELD('fo', 'Hej', 'ej', 'Heja', 'hej', 'foo');
            -> 0
    
  • FIND_IN_SET(str,strlist)

    Returns a value in the range of 1 to N if the string str is in the string list strlist consisting of N substrings. A string list is a string composed of substrings separated by , characters. If the first argument is a constant string and the second is a column of type SET, the FIND_IN_SET() function is optimized to use bit arithmetic. Returns 0 if str is not in strlist or if strlist is the empty string. Returns NULL if either argument is NULL. This function does not work properly if the first argument contains a comma (,) character.

    mysql> SELECT FIND_IN_SET('b','a,b,c,d');
            -> 2
    
  • FORMAT(X,D[,locale])

    Formats the number X to a format like '#,###,###.##', rounded to D decimal places, and returns the result as a string. If D is 0, the result has no decimal point or fractional part.

    The optional third parameter enables a locale to be specified to be used for the result number's decimal point, thousands separator, and grouping between separators. Permissible locale values are the same as the legal values for the lc_time_names system variable (see Section 10.7, “MySQL Server Locale Support”). If no locale is specified, the default is 'en_US'.

    mysql> SELECT FORMAT(12332.123456, 4);
            -> '12,332.1235'
    mysql> SELECT FORMAT(12332.1,4);
            -> '12,332.1000'
    mysql> SELECT FORMAT(12332.2,0);
            -> '12,332'
    mysql> SELECT FORMAT(12332.2,2,'de_DE');
            -> '12.332,20'
    
  • HEX(str), HEX(N)

    For a string argument str, HEX() returns a hexadecimal string representation of str where each byte of each character in str is converted to two hexadecimal digits. (Multibyte characters therefore become more than two digits.) The inverse of this operation is performed by the UNHEX() function.

    For a numeric argument N, HEX() returns a hexadecimal string representation of the value of N treated as a longlong (BIGINT) number. This is equivalent to CONV(N,10,16). The inverse of this operation is performed by CONV(HEX(N),16,10).

    mysql> SELECT 0x616263, HEX('abc'), UNHEX(HEX('abc'));
            -> 'abc', 616263, 'abc'
    mysql> SELECT HEX(255), CONV(HEX(255),16,10);
            -> 'FF', 255
    
  • INSERT(str,pos,len,newstr)

    Returns the string str, with the substring beginning at position pos and len characters long replaced by the string newstr. Returns the original string if pos is not within the length of the string. Replaces the rest of the string from position pos if len is not within the length of the rest of the string. Returns NULL if any argument is NULL.

    mysql> SELECT INSERT('Quadratic', 3, 4, 'What');
            -> 'QuWhattic'
    mysql> SELECT INSERT('Quadratic', -1, 4, 'What');
            -> 'Quadratic'
    mysql> SELECT INSERT('Quadratic', 3, 100, 'What');
            -> 'QuWhat'
    

    This function is multibyte safe.

  • INSTR(str,substr)

    Returns the position of the first occurrence of substring substr in string str. This is the same as the two-argument form of LOCATE(), except that the order of the arguments is reversed.

    mysql> SELECT INSTR('foobarbar', 'bar');
            -> 4
    mysql> SELECT INSTR('xbar', 'foobar');
            -> 0
    

    This function is multibyte safe, and is case sensitive only if at least one argument is a binary string.

  • LCASE(str)

    LCASE() is a synonym for LOWER().

  • LEFT(str,len)

    Returns the leftmost len characters from the string str, or NULL if any argument is NULL.

    mysql> SELECT LEFT('foobarbar', 5);
            -> 'fooba'
    

    This function is multibyte safe.

  • LENGTH(str)

    Returns the length of the string str, measured in bytes. A multibyte character counts as multiple bytes. This means that for a string containing five 2-byte characters, LENGTH() returns 10, whereas CHAR_LENGTH() returns 5.

    mysql> SELECT LENGTH('text');
            -> 4
    
    Note

    The Length() OpenGIS spatial function is named GLength() in MySQL.

  • LOAD_FILE(file_name)

    Reads the file and returns the file contents as a string. To use this function, the file must be located on the server host, you must specify the full path name to the file, and you must have the FILE privilege. The file must be readable by all and its size less than max_allowed_packet bytes. If the secure_file_priv system variable is set to a nonempty directory name, the file to be loaded must be located in that directory.

    If the file does not exist or cannot be read because one of the preceding conditions is not satisfied, the function returns NULL.

    The character_set_filesystem system variable controls interpretation of file names that are given as literal strings.

    mysql> UPDATE t
                SET blob_col=LOAD_FILE('/tmp/picture')
                WHERE id=1;
    
  • LOCATE(substr,str), LOCATE(substr,str,pos)

    The first syntax returns the position of the first occurrence of substring substr in string str. The second syntax returns the position of the first occurrence of substring substr in string str, starting at position pos. Returns 0 if substr is not in str.

    mysql> SELECT LOCATE('bar', 'foobarbar');
            -> 4
    mysql> SELECT LOCATE('xbar', 'foobar');
            -> 0
    mysql> SELECT LOCATE('bar', 'foobarbar', 5);
            -> 7
    

    This function is multibyte safe, and is case-sensitive only if at least one argument is a binary string.

  • LOWER(str)

    Returns the string str with all characters changed to lowercase according to the current character set mapping. The default is latin1 (cp1252 West European).

    mysql> SELECT LOWER('QUADRATICALLY');
            -> 'quadratically'
    

    LOWER() (and UPPER()) are ineffective when applied to binary strings (BINARY, VARBINARY, BLOB). To perform lettercase conversion, convert the string to a nonbinary string:

    mysql> SET @str = BINARY 'New York';
    mysql> SELECT LOWER(@str), LOWER(CONVERT(@str USING latin1));
    +-------------+-----------------------------------+
    | LOWER(@str) | LOWER(CONVERT(@str USING latin1)) |
    +-------------+-----------------------------------+
    | New York    | new york                          |
    +-------------+-----------------------------------+
    

    This function is multibyte safe.

  • LPAD(str,len,padstr)

    Returns the string str, left-padded with the string padstr to a length of len characters. If str is longer than len, the return value is shortened to len characters.

    mysql> SELECT LPAD('hi',4,'??');
            -> '??hi'
    mysql> SELECT LPAD('hi',1,'??');
            -> 'h'
    
  • LTRIM(str)

    Returns the string str with leading space characters removed.

    mysql> SELECT LTRIM('  barbar');
            -> 'barbar'
    

    This function is multibyte safe.

  • MAKE_SET(bits,str1,str2,...)

    Returns a set value (a string containing substrings separated by , characters) consisting of the strings that have the corresponding bit in bits set. str1 corresponds to bit 0, str2 to bit 1, and so on. NULL values in str1, str2, ... are not appended to the result.

    mysql> SELECT MAKE_SET(1,'a','b','c');
            -> 'a'
    mysql> SELECT MAKE_SET(1 | 4,'hello','nice','world');
            -> 'hello,world'
    mysql> SELECT MAKE_SET(1 | 4,'hello','nice',NULL,'world');
            -> 'hello'
    mysql> SELECT MAKE_SET(0,'a','b','c');
            -> ''
    
  • MID(str,pos,len)

    MID(str,pos,len) is a synonym for SUBSTRING(str,pos,len).

  • OCT(N)

    Returns a string representation of the octal value of N, where N is a longlong (BIGINT) number. This is equivalent to CONV(N,10,8). Returns NULL if N is NULL.

    mysql> SELECT OCT(12);
            -> '14'
    
  • OCTET_LENGTH(str)

    OCTET_LENGTH() is a synonym for LENGTH().

  • ORD(str)

    If the leftmost character of the string str is a multibyte character, returns the code for that character, calculated from the numeric values of its constituent bytes using this formula:

      (1st byte code)
    + (2nd byte code * 256)
    + (3rd byte code * 2562) ...
    

    If the leftmost character is not a multibyte character, ORD() returns the same value as the ASCII() function.

    mysql> SELECT ORD('2');
            -> 50
    
  • POSITION(substr IN str)

    POSITION(substr IN str) is a synonym for LOCATE(substr,str).

  • QUOTE(str)

    Quotes a string to produce a result that can be used as a properly escaped data value in an SQL statement. The string is returned enclosed by single quotation marks and with each instance of backslash (\), single quote ('), ASCII NUL, and Control+Z preceded by a backslash. If the argument is NULL, the return value is the word NULL without enclosing single quotation marks.

    mysql> SELECT QUOTE('Don\'t!');
            -> 'Don\'t!'
    mysql> SELECT QUOTE(NULL);
            -> NULL
    

    For comparison, see the quoting rules for literal strings and within the C API in Section 9.1.1, “String Literals”, and Section 23.8.7.53, “mysql_real_escape_string()”.

  • REPEAT(str,count)

    Returns a string consisting of the string str repeated count times. If count is less than 1, returns an empty string. Returns NULL if str or count are NULL.

    mysql> SELECT REPEAT('MySQL', 3);
            -> 'MySQLMySQLMySQL'
    
  • REPLACE(str,from_str,to_str)

    Returns the string str with all occurrences of the string from_str replaced by the string to_str. REPLACE() performs a case-sensitive match when searching for from_str.

    mysql> SELECT REPLACE('www.mysql.com', 'w', 'Ww');
            -> 'WwWwWw.mysql.com'
    

    This function is multibyte safe.

  • REVERSE(str)

    Returns the string str with the order of the characters reversed.

    mysql> SELECT REVERSE('abc');
            -> 'cba'
    

    This function is multibyte safe.

  • RIGHT(str,len)

    Returns the rightmost len characters from the string str, or NULL if any argument is NULL.

    mysql> SELECT RIGHT('foobarbar', 4);
            -> 'rbar'
    

    This function is multibyte safe.

  • RPAD(str,len,padstr)

    Returns the string str, right-padded with the string padstr to a length of len characters. If str is longer than len, the return value is shortened to len characters.

    mysql> SELECT RPAD('hi',5,'?');
            -> 'hi???'
    mysql> SELECT RPAD('hi',1,'?');
            -> 'h'
    

    This function is multibyte safe.

  • RTRIM(str)

    Returns the string str with trailing space characters removed.

    mysql> SELECT RTRIM('barbar   ');
            -> 'barbar'
    

    This function is multibyte safe.

  • SOUNDEX(str)

    Returns a soundex string from str. Two strings that sound almost the same should have identical soundex strings. A standard soundex string is four characters long, but the SOUNDEX() function returns an arbitrarily long string. You can use SUBSTRING() on the result to get a standard soundex string. All nonalphabetic characters in str are ignored. All international alphabetic characters outside the A-Z range are treated as vowels.

    Important

    When using SOUNDEX(), you should be aware of the following limitations:

    • This function, as currently implemented, is intended to work well with strings that are in the English language only. Strings in other languages may not produce reliable results.

    • This function is not guaranteed to provide consistent results with strings that use multibyte character sets, including utf-8.

      We hope to remove these limitations in a future release. See Bug #22638 for more information.

    mysql> SELECT SOUNDEX('Hello');
            -> 'H400'
    mysql> SELECT SOUNDEX('Quadratically');
            -> 'Q36324'
    
    Note

    This function implements the original Soundex algorithm, not the more popular enhanced version (also described by D. Knuth). The difference is that original version discards vowels first and duplicates second, whereas the enhanced version discards duplicates first and vowels second.

  • expr1 SOUNDS LIKE expr2

    This is the same as SOUNDEX(expr1) = SOUNDEX(expr2).

  • SPACE(N)

    Returns a string consisting of N space characters.

    mysql> SELECT SPACE(6);
            -> '      '
    
  • SUBSTR(str,pos), SUBSTR(str FROM pos), SUBSTR(str,pos,len), SUBSTR(str FROM pos FOR len)

    SUBSTR() is a synonym for SUBSTRING().

  • SUBSTRING(str,pos), SUBSTRING(str FROM pos), SUBSTRING(str,pos,len), SUBSTRING(str FROM pos FOR len)

    The forms without a len argument return a substring from string str starting at position pos. The forms with a len argument return a substring len characters long from string str, starting at position pos. The forms that use FROM are standard SQL syntax. It is also possible to use a negative value for pos. In this case, the beginning of the substring is pos characters from the end of the string, rather than the beginning. A negative value may be used for pos in any of the forms of this function.

    For all forms of SUBSTRING(), the position of the first character in the string from which the substring is to be extracted is reckoned as 1.

    mysql> SELECT SUBSTRING('Quadratically',5);
            -> 'ratically'
    mysql> SELECT SUBSTRING('foobarbar' FROM 4);
            -> 'barbar'
    mysql> SELECT SUBSTRING('Quadratically',5,6);
            -> 'ratica'
    mysql> SELECT SUBSTRING('Sakila', -3);
            -> 'ila'
    mysql> SELECT SUBSTRING('Sakila', -5, 3);
            -> 'aki'
    mysql> SELECT SUBSTRING('Sakila' FROM -4 FOR 2);
            -> 'ki'
    

    This function is multibyte safe.

    If len is less than 1, the result is the empty string.

  • SUBSTRING_INDEX(str,delim,count)

    Returns the substring from string str before count occurrences of the delimiter delim. If count is positive, everything to the left of the final delimiter (counting from the left) is returned. If count is negative, everything to the right of the final delimiter (counting from the right) is returned. SUBSTRING_INDEX() performs a case-sensitive match when searching for delim.

    mysql> SELECT SUBSTRING_INDEX('www.mysql.com', '.', 2);
            -> 'www.mysql'
    mysql> SELECT SUBSTRING_INDEX('www.mysql.com', '.', -2);
            -> 'mysql.com'
    

    This function is multibyte safe.

  • TRIM([{BOTH | LEADING | TRAILING} [remstr] FROM] str), TRIM([remstr FROM] str)

    Returns the string str with all remstr prefixes or suffixes removed. If none of the specifiers BOTH, LEADING, or TRAILING is given, BOTH is assumed. remstr is optional and, if not specified, spaces are removed.

    mysql> SELECT TRIM('  bar   ');
            -> 'bar'
    mysql> SELECT TRIM(LEADING 'x' FROM 'xxxbarxxx');
            -> 'barxxx'
    mysql> SELECT TRIM(BOTH 'x' FROM 'xxxbarxxx');
            -> 'bar'
    mysql> SELECT TRIM(TRAILING 'xyz' FROM 'barxxyz');
            -> 'barx'
    

    This function is multibyte safe.

  • UCASE(str)

    UCASE() is a synonym for UPPER().

  • UNHEX(str)

    For a string argument str, UNHEX(str) interprets each pair of characters in the argument as a hexadecimal number and converts it to the byte represented by the number. The return value is a binary string.

    mysql> SELECT UNHEX('4D7953514C');
            -> 'MySQL'
    mysql> SELECT 0x4D7953514C;
            -> 'MySQL'
    mysql> SELECT UNHEX(HEX('string'));
            -> 'string'
    mysql> SELECT HEX(UNHEX('1267'));
            -> '1267'
    

    The characters in the argument string must be legal hexadecimal digits: '0' .. '9', 'A' .. 'F', 'a' .. 'f'. If the argument contains any nonhexadecimal digits, the result is NULL:

    mysql> SELECT UNHEX('GG');
    +-------------+
    | UNHEX('GG') |
    +-------------+
    | NULL        |
    +-------------+
    

    A NULL result can occur if the argument to UNHEX() is a BINARY column, because values are padded with 0x00 bytes when stored but those bytes are not stripped on retrieval. For example, '41' is stored into a CHAR(3) column as '41 ' and retrieved as '41' (with the trailing pad space stripped), so UNHEX() for the column value returns 'A'. By contrast '41' is stored into a BINARY(3) column as '41\0' and retrieved as '41\0' (with the trailing pad 0x00 byte not stripped). '\0' is not a legal hexadecimal digit, so UNHEX() for the column value returns NULL.

    For a numeric argument N, the inverse of HEX(N) is not performed by UNHEX(). Use CONV(HEX(N),16,10) instead. See the description of HEX().

  • UPPER(str)

    Returns the string str with all characters changed to uppercase according to the current character set mapping. The default is latin1 (cp1252 West European).

    mysql> SELECT UPPER('Hej');
            -> 'HEJ'
    

    See the description of LOWER() for information that also applies to UPPER(), such as information about how to perform lettercase conversion of binary strings (BINARY, VARBINARY, BLOB) for which these functions are ineffective.

    This function is multibyte safe.

12.5.1 String Comparison Functions

Table 12.8 String Comparison Operators

NameDescription
LIKESimple pattern matching
NOT LIKENegation of simple pattern matching
STRCMP()Compare two strings

If a string function is given a binary string as an argument, the resulting string is also a binary string. A number converted to a string is treated as a binary string. This affects only comparisons.

Normally, if any expression in a string comparison is case sensitive, the comparison is performed in case-sensitive fashion.

  • expr LIKE pat [ESCAPE 'escape_char']

    Pattern matching using SQL simple regular expression comparison. Returns 1 (TRUE) or 0 (FALSE). If either expr or pat is NULL, the result is NULL.

    The pattern need not be a literal string. For example, it can be specified as a string expression or table column.

    Per the SQL standard, LIKE performs matching on a per-character basis, thus it can produce results different from the = comparison operator:

    mysql> SELECT 'ä' LIKE 'ae' COLLATE latin1_german2_ci;
    +-----------------------------------------+
    | 'ä' LIKE 'ae' COLLATE latin1_german2_ci |
    +-----------------------------------------+
    |                                       0 |
    +-----------------------------------------+
    mysql> SELECT 'ä' = 'ae' COLLATE latin1_german2_ci;
    +--------------------------------------+
    | 'ä' = 'ae' COLLATE latin1_german2_ci |
    +--------------------------------------+
    |                                    1 |
    +--------------------------------------+
    

    In particular, trailing spaces are significant, which is not true for CHAR or VARCHAR comparisons performed with the = operator:

    mysql> SELECT 'a' = 'a ', 'a' LIKE 'a ';
    +------------+---------------+
    | 'a' = 'a ' | 'a' LIKE 'a ' |
    +------------+---------------+
    |          1 |             0 |
    +------------+---------------+
    1 row in set (0.00 sec)
    

    With LIKE you can use the following two wildcard characters in the pattern.

    CharacterDescription
    %Matches any number of characters, even zero characters
    _Matches exactly one character
    mysql> SELECT 'David!' LIKE 'David_';
            -> 1
    mysql> SELECT 'David!' LIKE '%D%v%';
            -> 1
    

    To test for literal instances of a wildcard character, precede it by the escape character. If you do not specify the ESCAPE character, \ is assumed.

    StringDescription
    \%Matches one % character
    \_Matches one _ character
    mysql> SELECT 'David!' LIKE 'David\_';
            -> 0
    mysql> SELECT 'David_' LIKE 'David\_';
            -> 1
    

    To specify a different escape character, use the ESCAPE clause:

    mysql> SELECT 'David_' LIKE 'David|_' ESCAPE '|';
            -> 1
    

    The escape sequence should be empty or one character long. The expression must evaluate as a constant at execution time. If the NO_BACKSLASH_ESCAPES SQL mode is enabled, the sequence cannot be empty.

    The following two statements illustrate that string comparisons are not case sensitive unless one of the operands is a binary string:

    mysql> SELECT 'abc' LIKE 'ABC';
            -> 1
    mysql> SELECT 'abc' LIKE BINARY 'ABC';
            -> 0
    

    In MySQL, LIKE is permitted on numeric expressions. (This is an extension to the standard SQL LIKE.)

    mysql> SELECT 10 LIKE '1%';
            -> 1
    
    Note

    Because MySQL uses C escape syntax in strings (for example, \n to represent a newline character), you must double any \ that you use in LIKE strings. For example, to search for \n, specify it as \\n. To search for \, specify it as \\\\; this is because the backslashes are stripped once by the parser and again when the pattern match is made, leaving a single backslash to be matched against.

    Exception: At the end of the pattern string, backslash can be specified as \\. At the end of the string, backslash stands for itself because there is nothing following to escape. Suppose that a table contains the following values:

    mysql> SELECT filename FROM t1;
    +--------------+
    | filename     |
    +--------------+
    | C:           | 
    | C:\          | 
    | C:\Programs  | 
    | C:\Programs\ | 
    +--------------+
    

    To test for values that end with backslash, you can match the values using either of the following patterns:

    mysql> SELECT filename, filename LIKE '%\\' FROM t1;
    +--------------+---------------------+
    | filename     | filename LIKE '%\\' |
    +--------------+---------------------+
    | C:           |                   0 | 
    | C:\          |                   1 | 
    | C:\Programs  |                   0 | 
    | C:\Programs\ |                   1 | 
    +--------------+---------------------+
    
    mysql> SELECT filename, filename LIKE '%\\\\' FROM t1;
    +--------------+-----------------------+
    | filename     | filename LIKE '%\\\\' |
    +--------------+-----------------------+
    | C:           |                     0 | 
    | C:\          |                     1 | 
    | C:\Programs  |                     0 | 
    | C:\Programs\ |                     1 | 
    +--------------+-----------------------+
    
  • expr NOT LIKE pat [ESCAPE 'escape_char']

    This is the same as NOT (expr LIKE pat [ESCAPE 'escape_char']).

    Note

    Aggregate queries involving NOT LIKE comparisons with columns containing NULL may yield unexpected results. For example, consider the following table and data:

    CREATE TABLE foo (bar VARCHAR(10));
    
    INSERT INTO foo VALUES (NULL), (NULL);
    

    The query SELECT COUNT(*) FROM foo WHERE bar LIKE '%baz%'; returns 0. You might assume that SELECT COUNT(*) FROM foo WHERE bar NOT LIKE '%baz%'; would return 2. However, this is not the case: The second query returns 0. This is because NULL NOT LIKE expr always returns NULL, regardless of the value of expr. The same is true for aggregate queries involving NULL and comparisons using NOT RLIKE or NOT REGEXP. In such cases, you must test explicitly for NOT NULL using OR (and not AND), as shown here:

    SELECT COUNT(*) FROM foo WHERE bar NOT LIKE '%baz%' OR bar IS NULL;
    
  • STRCMP(expr1,expr2)

    STRCMP() returns 0 if the strings are the same, -1 if the first argument is smaller than the second according to the current sort order, and 1 otherwise.

    mysql> SELECT STRCMP('text', 'text2');
            -> -1
    mysql> SELECT STRCMP('text2', 'text');
            -> 1
    mysql> SELECT STRCMP('text', 'text');
            -> 0
    

    STRCMP() performs the comparison using the collation of the arguments.

    mysql> SET @s1 = _latin1 'x' COLLATE latin1_general_ci;
    mysql> SET @s2 = _latin1 'X' COLLATE latin1_general_ci;
    mysql> SET @s3 = _latin1 'x' COLLATE latin1_general_cs;
    mysql> SET @s4 = _latin1 'X' COLLATE latin1_general_cs;
    mysql> SELECT STRCMP(@s1, @s2), STRCMP(@s3, @s4);
    +------------------+------------------+
    | STRCMP(@s1, @s2) | STRCMP(@s3, @s4) |
    +------------------+------------------+
    |                0 |                1 |
    +------------------+------------------+
    

    If the collations are incompatible, one of the arguments must be converted to be compatible with the other. See Section 10.1.7.5, “Collation of Expressions”.

    
    mysql> SELECT STRCMP(@s1, @s3);
    ERROR 1267 (HY000): Illegal mix of collations (latin1_general_ci,IMPLICIT) and (latin1_general_cs,IMPLICIT) for operation 'strcmp'
    mysql> SELECT STRCMP(@s1, @s3 COLLATE latin1_general_ci);
    +--------------------------------------------+
    | STRCMP(@s1, @s3 COLLATE latin1_general_ci) |
    +--------------------------------------------+
    |                                          0 |
    +--------------------------------------------+
    

12.5.2 Regular Expressions

Table 12.9 String Regular Expression Operators

NameDescription
NOT REGEXPNegation of REGEXP
REGEXPPattern matching using regular expressions
RLIKESynonym for REGEXP

A regular expression is a powerful way of specifying a pattern for a complex search.

MySQL uses Henry Spencer's implementation of regular expressions, which is aimed at conformance with POSIX 1003.2. MySQL uses the extended version to support pattern-matching operations performed with the REGEXP operator in SQL statements.

This section summarizes, with examples, the special characters and constructs that can be used in MySQL for REGEXP operations. It does not contain all the details that can be found in Henry Spencer's regex(7) manual page. That manual page is included in MySQL source distributions, in the regex.7 file under the regex directory. See also Section 3.3.4.7, “Pattern Matching”.

Regular Expression Operators

  • expr NOT REGEXP pat, expr NOT RLIKE pat

    This is the same as NOT (expr REGEXP pat).

  • expr REGEXP pat, expr RLIKE pat

    Performs a pattern match of a string expression expr against a pattern pat. The pattern can be an extended regular expression, the syntax for which is discussed later in this section. Returns 1 if expr matches pat; otherwise it returns 0. If either expr or pat is NULL, the result is NULL. RLIKE is a synonym for REGEXP, provided for mSQL compatibility.

    The pattern need not be a literal string. For example, it can be specified as a string expression or table column.

    Note

    Because MySQL uses the C escape syntax in strings (for example, \n to represent the newline character), you must double any \ that you use in your REGEXP strings.

    REGEXP is not case sensitive, except when used with binary strings.

    mysql> SELECT 'Monty!' REGEXP '.*';
            -> 1
    mysql> SELECT 'new*\n*line' REGEXP 'new\\*.\\*line';
            -> 1
    mysql> SELECT 'a' REGEXP 'A', 'a' REGEXP BINARY 'A';
            -> 1  0
    mysql> SELECT 'a' REGEXP '^[a-d]';
            -> 1
    

    REGEXP and RLIKE use the character set and collations of the arguments when deciding the type of a character and performing the comparison. If the arguments have different character sets or collations, coercibility rules apply as described in Section 10.1.7.5, “Collation of Expressions”.

    Warning

    The REGEXP and RLIKE operators work in byte-wise fashion, so they are not multibyte safe and may produce unexpected results with multibyte character sets. In addition, these operators compare characters by their byte values and accented characters may not compare as equal even if a given collation treats them as equal.

Syntax of Regular Expressions

A regular expression describes a set of strings. The simplest regular expression is one that has no special characters in it. For example, the regular expression hello matches hello and nothing else.

Nontrivial regular expressions use certain special constructs so that they can match more than one string. For example, the regular expression hello|word matches either the string hello or the string word.

As a more complex example, the regular expression B[an]*s matches any of the strings Bananas, Baaaaas, Bs, and any other string starting with a B, ending with an s, and containing any number of a or n characters in between.

A regular expression for the REGEXP operator may use any of the following special characters and constructs:

  • ^

    Match the beginning of a string.

    mysql> SELECT 'fo\nfo' REGEXP '^fo$';                   -> 0
    mysql> SELECT 'fofo' REGEXP '^fo';                      -> 1
    
  • $

    Match the end of a string.

    mysql> SELECT 'fo\no' REGEXP '^fo\no$';                 -> 1
    mysql> SELECT 'fo\no' REGEXP '^fo$';                    -> 0
    
  • .

    Match any character (including carriage return and newline).

    mysql> SELECT 'fofo' REGEXP '^f.*$';                    -> 1
    mysql> SELECT 'fo\r\nfo' REGEXP '^f.*$';                -> 1
    
  • a*

    Match any sequence of zero or more a characters.

    mysql> SELECT 'Ban' REGEXP '^Ba*n';                     -> 1
    mysql> SELECT 'Baaan' REGEXP '^Ba*n';                   -> 1
    mysql> SELECT 'Bn' REGEXP '^Ba*n';                      -> 1
    
  • a+

    Match any sequence of one or more a characters.

    mysql> SELECT 'Ban' REGEXP '^Ba+n';                     -> 1
    mysql> SELECT 'Bn' REGEXP '^Ba+n';                      -> 0
    
  • a?

    Match either zero or one a character.

    mysql> SELECT 'Bn' REGEXP '^Ba?n';                      -> 1
    mysql> SELECT 'Ban' REGEXP '^Ba?n';                     -> 1
    mysql> SELECT 'Baan' REGEXP '^Ba?n';                    -> 0
    
  • de|abc

    Match either of the sequences de or abc.

    mysql> SELECT 'pi' REGEXP 'pi|apa';                     -> 1
    mysql> SELECT 'axe' REGEXP 'pi|apa';                    -> 0
    mysql> SELECT 'apa' REGEXP 'pi|apa';                    -> 1
    mysql> SELECT 'apa' REGEXP '^(pi|apa)$';                -> 1
    mysql> SELECT 'pi' REGEXP '^(pi|apa)$';                 -> 1
    mysql> SELECT 'pix' REGEXP '^(pi|apa)$';                -> 0
    
  • (abc)*

    Match zero or more instances of the sequence abc.

    mysql> SELECT 'pi' REGEXP '^(pi)*$';                    -> 1
    mysql> SELECT 'pip' REGEXP '^(pi)*$';                   -> 0
    mysql> SELECT 'pipi' REGEXP '^(pi)*$';                  -> 1
    
  • {1}, {2,3}

    {n} or {m,n} notation provides a more general way of writing regular expressions that match many occurrences of the previous atom (or piece) of the pattern. m and n are integers.

    • a*

      Can be written as a{0,}.

    • a+

      Can be written as a{1,}.

    • a?

      Can be written as a{0,1}.

    To be more precise, a{n} matches exactly n instances of a. a{n,} matches n or more instances of a. a{m,n} matches m through n instances of a, inclusive.

    m and n must be in the range from 0 to RE_DUP_MAX (default 255), inclusive. If both m and n are given, m must be less than or equal to n.

    mysql> SELECT 'abcde' REGEXP 'a[bcd]{2}e';              -> 0
    mysql> SELECT 'abcde' REGEXP 'a[bcd]{3}e';              -> 1
    mysql> SELECT 'abcde' REGEXP 'a[bcd]{1,10}e';           -> 1
    
  • [a-dX], [^a-dX]

    Matches any character that is (or is not, if ^ is used) either a, b, c, d or X. A - character between two other characters forms a range that matches all characters from the first character to the second. For example, [0-9] matches any decimal digit. To include a literal ] character, it must immediately follow the opening bracket [. To include a literal - character, it must be written first or last. Any character that does not have a defined special meaning inside a [] pair matches only itself.

    mysql> SELECT 'aXbc' REGEXP '[a-dXYZ]';                 -> 1
    mysql> SELECT 'aXbc' REGEXP '^[a-dXYZ]$';               -> 0
    mysql> SELECT 'aXbc' REGEXP '^[a-dXYZ]+$';              -> 1
    mysql> SELECT 'aXbc' REGEXP '^[^a-dXYZ]+$';             -> 0
    mysql> SELECT 'gheis' REGEXP '^[^a-dXYZ]+$';            -> 1
    mysql> SELECT 'gheisa' REGEXP '^[^a-dXYZ]+$';           -> 0
    
  • [.characters.]

    Within a bracket expression (written using [ and ]), matches the sequence of characters of that collating element. characters is either a single character or a character name like newline. The following table lists the permissible character names.

    The following table shows the permissible character names and the characters that they match. For characters given as numeric values, the values are represented in octal.

    NameCharacterNameCharacter
    NUL0SOH001
    STX002ETX003
    EOT004ENQ005
    ACK006BEL007
    alert007BS010
    backspace'\b'HT011
    tab'\t'LF012
    newline'\n'VT013
    vertical-tab'\v'FF014
    form-feed'\f'CR015
    carriage-return'\r'SO016
    SI017DLE020
    DC1021DC2022
    DC3023DC4024
    NAK025SYN026
    ETB027CAN030
    EM031SUB032
    ESC033IS4034
    FS034IS3035
    GS035IS2036
    RS036IS1037
    US037space' '
    exclamation-mark'!'quotation-mark'"'
    number-sign'#'dollar-sign'$'
    percent-sign'%'ampersand'&'
    apostrophe'\''left-parenthesis'('
    right-parenthesis')'asterisk'*'
    plus-sign'+'comma','
    hyphen'-'hyphen-minus'-'
    period'.'full-stop'.'
    slash'/'solidus'/'
    zero'0'one'1'
    two'2'three'3'
    four'4'five'5'
    six'6'seven'7'
    eight'8'nine'9'
    colon':'semicolon';'
    less-than-sign'<'equals-sign'='
    greater-than-sign'>'question-mark'?'
    commercial-at'@'left-square-bracket'['
    backslash'\\'reverse-solidus'\\'
    right-square-bracket']'circumflex'^'
    circumflex-accent'^'underscore'_'
    low-line'_'grave-accent'`'
    left-brace'{'left-curly-bracket'{'
    vertical-line'|'right-brace'}'
    right-curly-bracket'}'tilde'~'
    DEL177  
    mysql> SELECT '~' REGEXP '[[.~.]]';                     -> 1
    mysql> SELECT '~' REGEXP '[[.tilde.]]';                 -> 1
    
  • [=character_class=]

    Within a bracket expression (written using [ and ]), [=character_class=] represents an equivalence class. It matches all characters with the same collation value, including itself. For example, if o and (+) are the members of an equivalence class, [[=o=]], [[=(+)=]], and [o(+)] are all synonymous. An equivalence class may not be used as an endpoint of a range.

  • [:character_class:]

    Within a bracket expression (written using [ and ]), [:character_class:] represents a character class that matches all characters belonging to that class. The following table lists the standard class names. These names stand for the character classes defined in the ctype(3) manual page. A particular locale may provide other class names. A character class may not be used as an endpoint of a range.

    Character Class NameMeaning
    alnumAlphanumeric characters
    alphaAlphabetic characters
    blankWhitespace characters
    cntrlControl characters
    digitDigit characters
    graphGraphic characters
    lowerLowercase alphabetic characters
    printGraphic or space characters
    punctPunctuation characters
    spaceSpace, tab, newline, and carriage return
    upperUppercase alphabetic characters
    xdigitHexadecimal digit characters
    mysql> SELECT 'justalnums' REGEXP '[[:alnum:]]+';       -> 1
    mysql> SELECT '!!' REGEXP '[[:alnum:]]+';               -> 0
    
  • [[:<:]], [[:>:]]

    These markers stand for word boundaries. They match the beginning and end of words, respectively. A word is a sequence of word characters that is not preceded by or followed by word characters. A word character is an alphanumeric character in the alnum class or an underscore (_).

    mysql> SELECT 'a word a' REGEXP '[[:<:]]word[[:>:]]';   -> 1
    mysql> SELECT 'a xword a' REGEXP '[[:<:]]word[[:>:]]';  -> 0
    

To use a literal instance of a special character in a regular expression, precede it by two backslash (\) characters. The MySQL parser interprets one of the backslashes, and the regular expression library interprets the other. For example, to match the string 1+2 that contains the special + character, only the last of the following regular expressions is the correct one:

mysql> SELECT '1+2' REGEXP '1+2';                       -> 0
mysql> SELECT '1+2' REGEXP '1\+2';                      -> 0
mysql> SELECT '1+2' REGEXP '1\\+2';                     -> 1

12.6 Numeric Functions and Operators

Table 12.10 Numeric Functions and Operators

NameDescription
ABS()Return the absolute value
ACOS()Return the arc cosine
ASIN()Return the arc sine
ATAN2(), ATAN()Return the arc tangent of the two arguments
ATAN()Return the arc tangent
CEIL()Return the smallest integer value not less than the argument
CEILING()Return the smallest integer value not less than the argument
CONV()Convert numbers between different number bases
COS()Return the cosine
COT()Return the cotangent
CRC32()Compute a cyclic redundancy check value
DEGREES()Convert radians to degrees
DIVInteger division
/Division operator
EXP()Raise to the power of
FLOOR()Return the largest integer value not greater than the argument
LN()Return the natural logarithm of the argument
LOG10()Return the base-10 logarithm of the argument
LOG2()Return the base-2 logarithm of the argument
LOG()Return the natural logarithm of the first argument
-Minus operator
MOD()Return the remainder
% or MODModulo operator
PI()Return the value of pi
+Addition operator
POW()Return the argument raised to the specified power
POWER()Return the argument raised to the specified power
RADIANS()Return argument converted to radians
RAND()Return a random floating-point value
ROUND()Round the argument
SIGN()Return the sign of the argument
SIN()Return the sine of the argument
SQRT()Return the square root of the argument
TAN()Return the tangent of the argument
*Multiplication operator
TRUNCATE()Truncate to specified number of decimal places
-Change the sign of the argument

12.6.1 Arithmetic Operators

Table 12.11 Arithmetic Operators

NameDescription
DIVInteger division
/Division operator
-Minus operator
% or MODModulo operator
+Addition operator
*Multiplication operator
-Change the sign of the argument

The usual arithmetic operators are available. The result is determined according to the following rules:

  • In the case of -, +, and *, the result is calculated with BIGINT (64-bit) precision if both operands are integers.

  • If both operands are integers and any of them are unsigned, the result is an unsigned integer. For subtraction, if the NO_UNSIGNED_SUBTRACTION SQL mode is enabled, the result is signed even if any operand is unsigned.

  • If any of the operands of a +, -, /, *, % is a real or string value, the precision of the result is the precision of the operand with the maximum precision.

  • In division performed with /, the scale of the result when using two exact-value operands is the scale of the first operand plus the value of the div_precision_increment system variable (which is 4 by default). For example, the result of the expression 5.05 / 0.014 has a scale of six decimal places (360.714286).

These rules are applied for each operation, such that nested calculations imply the precision of each component. Hence, (14620 / 9432456) / (24250 / 9432456), resolves first to (0.0014) / (0.0026), with the final result having 8 decimal places (0.60288653).

Because of these rules and the way they are applied, care should be taken to ensure that components and subcomponents of a calculation use the appropriate level of precision. See Section 12.10, “Cast Functions and Operators”.

For information about handling of overflow in numeric expression evaluation, see Section 11.2.6, “Out-of-Range and Overflow Handling”.

Arithmetic operators apply to numbers. For other types of values, alternative operations may be available. For example, to add date values, use DATE_ADD(); see Section 12.7, “Date and Time Functions”.

  • +

    Addition:

    mysql> SELECT 3+5;
            -> 8
    
  • -

    Subtraction:

    mysql> SELECT 3-5;
            -> -2
    
  • -

    Unary minus. This operator changes the sign of the operand.

    mysql> SELECT - 2;
            -> -2
    
    Note

    If this operator is used with a BIGINT, the return value is also a BIGINT. This means that you should avoid using - on integers that may have the value of –263.

  • *

    Multiplication:

    mysql> SELECT 3*5;
            -> 15
    mysql> SELECT 18014398509481984*18014398509481984.0;
            -> 324518553658426726783156020576256.0
    mysql> SELECT 18014398509481984*18014398509481984;
            -> out-of-range error
    

    The last expression produces an error because the result of the integer multiplication exceeds the 64-bit range of BIGINT calculations. (See Section 11.2, “Numeric Types”.)

  • /

    Division:

    mysql> SELECT 3/5;
            -> 0.60
    

    Division by zero produces a NULL result:

    mysql> SELECT 102/(1-1);
            -> NULL
    

    A division is calculated with BIGINT arithmetic only if performed in a context where its result is converted to an integer.

  • DIV

    Integer division. Similar to FLOOR(), but is safe with BIGINT values.

    As of MySQL 5.5.3, if either operand has a noninteger type, the operands are converted to DECIMAL and divided using DECIMAL arithmetic before converting the result to BIGINT. If the result exceeds BIGINT range, an error occurs. Before MySQL 5.5.3, incorrect results may occur for noninteger operands that exceed BIGINT range.

    mysql> SELECT 5 DIV 2;
            -> 2
    
  • N % M, N MOD M

    Modulo operation. Returns the remainder of N divided by M. For more information, see the description for the MOD() function in Section 12.6.2, “Mathematical Functions”.

12.6.2 Mathematical Functions

Table 12.12 Mathematical Functions

NameDescription
ABS()Return the absolute value
ACOS()Return the arc cosine
ASIN()Return the arc sine
ATAN2(), ATAN()Return the arc tangent of the two arguments
ATAN()Return the arc tangent
CEIL()Return the smallest integer value not less than the argument
CEILING()Return the smallest integer value not less than the argument
CONV()Convert numbers between different number bases
COS()Return the cosine
COT()Return the cotangent
CRC32()Compute a cyclic redundancy check value
DEGREES()Convert radians to degrees
EXP()Raise to the power of
FLOOR()Return the largest integer value not greater than the argument
LN()Return the natural logarithm of the argument
LOG10()Return the base-10 logarithm of the argument
LOG2()Return the base-2 logarithm of the argument
LOG()Return the natural logarithm of the first argument
MOD()Return the remainder
PI()Return the value of pi
POW()Return the argument raised to the specified power
POWER()Return the argument raised to the specified power
RADIANS()Return argument converted to radians
RAND()Return a random floating-point value
ROUND()Round the argument
SIGN()Return the sign of the argument
SIN()Return the sine of the argument
SQRT()Return the square root of the argument
TAN()Return the tangent of the argument
TRUNCATE()Truncate to specified number of decimal places

All mathematical functions return NULL in the event of an error.

  • ABS(X)

    Returns the absolute value of X.

    mysql> SELECT ABS(2);
            -> 2
    mysql> SELECT ABS(-32);
            -> 32
    

    This function is safe to use with BIGINT values.

  • ACOS(X)

    Returns the arc cosine of X, that is, the value whose cosine is X. Returns NULL if X is not in the range -1 to 1.

    mysql> SELECT ACOS(1);
            -> 0
    mysql> SELECT ACOS(1.0001);
            -> NULL
    mysql> SELECT ACOS(0);
            -> 1.5707963267949
    
  • ASIN(X)

    Returns the arc sine of X, that is, the value whose sine is X. Returns NULL if X is not in the range -1 to 1.

    mysql> SELECT ASIN(0.2);
            -> 0.20135792079033
    mysql> SELECT ASIN('foo');
    
    +-------------+
    | ASIN('foo') |
    +-------------+
    |           0 |
    +-------------+
    1 row in set, 1 warning (0.00 sec)
    
    mysql> SHOW WARNINGS;
    +---------+------+-----------------------------------------+
    | Level   | Code | Message                                 |
    +---------+------+-----------------------------------------+
    | Warning | 1292 | Truncated incorrect DOUBLE value: 'foo' |
    +---------+------+-----------------------------------------+
    
  • ATAN(X)

    Returns the arc tangent of X, that is, the value whose tangent is X.

    mysql> SELECT ATAN(2);
            -> 1.1071487177941
    mysql> SELECT ATAN(-2);
            -> -1.1071487177941
    
  • ATAN(Y,X), ATAN2(Y,X)

    Returns the arc tangent of the two variables X and Y. It is similar to calculating the arc tangent of Y / X, except that the signs of both arguments are used to determine the quadrant of the result.

    mysql> SELECT ATAN(-2,2);
            -> -0.78539816339745
    mysql> SELECT ATAN2(PI(),0);
            -> 1.5707963267949
    
  • CEIL(X)

    CEIL() is a synonym for CEILING().

  • CEILING(X)

    Returns the smallest integer value not less than X.

    mysql> SELECT CEILING(1.23);
            -> 2
    mysql> SELECT CEILING(-1.23);
            -> -1
    

    For exact-value numeric arguments, the return value has an exact-value numeric type. For string or floating-point arguments, the return value has a floating-point type.

  • CONV(N,from_base,to_base)

    Converts numbers between different number bases. Returns a string representation of the number N, converted from base from_base to base to_base. Returns NULL if any argument is NULL. The argument N is interpreted as an integer, but may be specified as an integer or a string. The minimum base is 2 and the maximum base is 36. If to_base is a negative number, N is regarded as a signed number. Otherwise, N is treated as unsigned. CONV() works with 64-bit precision.

    mysql> SELECT CONV('a',16,2);
            -> '1010'
    mysql> SELECT CONV('6E',18,8);
            -> '172'
    mysql> SELECT CONV(-17,10,-18);
            -> '-H'
    mysql> SELECT CONV(10+'10'+'10'+0xa,10,10);
            -> '40'
    
  • COS(X)

    Returns the cosine of X, where X is given in radians.

    mysql> SELECT COS(PI());
            -> -1
    
  • COT(X)

    Returns the cotangent of X.

    mysql> SELECT COT(12);
            -> -1.5726734063977
    mysql> SELECT COT(0);
            -> NULL
    
  • CRC32(expr)

    Computes a cyclic redundancy check value and returns a 32-bit unsigned value. The result is NULL if the argument is NULL. The argument is expected to be a string and (if possible) is treated as one if it is not.

    mysql> SELECT CRC32('MySQL');
            -> 3259397556
    mysql> SELECT CRC32('mysql');
            -> 2501908538
    
  • DEGREES(X)

    Returns the argument X, converted from radians to degrees.

    mysql> SELECT DEGREES(PI());
            -> 180
    mysql> SELECT DEGREES(PI() / 2);
            -> 90
    
  • EXP(X)

    Returns the value of e (the base of natural logarithms) raised to the power of X. The inverse of this function is LOG() (using a single argument only) or LN().

    mysql> SELECT EXP(2);
            -> 7.3890560989307
    mysql> SELECT EXP(-2);
            -> 0.13533528323661
    mysql> SELECT EXP(0);
            -> 1
    
  • FLOOR(X)

    Returns the largest integer value not greater than X.

    mysql> SELECT FLOOR(1.23);
            -> 1
    mysql> SELECT FLOOR(-1.23);
            -> -2
    

    For exact-value numeric arguments, the return value has an exact-value numeric type. For string or floating-point arguments, the return value has a floating-point type.

  • FORMAT(X,D)

    Formats the number X to a format like '#,###,###.##', rounded to D decimal places, and returns the result as a string. For details, see Section 12.5, “String Functions”.

  • HEX(N_or_S)

    This function can be used to obtain a hexadecimal representation of a decimal number or a string; the manner in which it does so varies according to the argument's type. See this function's description in Section 12.5, “String Functions”, for details.

  • LN(X)

    Returns the natural logarithm of X; that is, the base-e logarithm of X. If X is less than or equal to 0, then NULL is returned.

    mysql> SELECT LN(2);
            -> 0.69314718055995
    mysql> SELECT LN(-2);
            -> NULL
    

    This function is synonymous with LOG(X). The inverse of this function is the EXP() function.

  • LOG(X), LOG(B,X)

    If called with one parameter, this function returns the natural logarithm of X. If X is less than or equal to 0, then NULL is returned.

    The inverse of this function (when called with a single argument) is the EXP() function.

    mysql> SELECT LOG(2);
            -> 0.69314718055995
    mysql> SELECT LOG(-2);
            -> NULL
    

    If called with two parameters, this function returns the logarithm of X to the base B. If X is less than or equal to 0, or if B is less than or equal to 1, then NULL is returned.

    mysql> SELECT LOG(2,65536);
            -> 16
    mysql> SELECT LOG(10,100);
            -> 2
    mysql> SELECT LOG(1,100);
            -> NULL
    

    LOG(B,X) is equivalent to LOG(X) / LOG(B).

  • LOG2(X)

    Returns the base-2 logarithm of X.

    mysql> SELECT LOG2(65536);
            -> 16
    mysql> SELECT LOG2(-100);
            -> NULL
    

    LOG2() is useful for finding out how many bits a number requires for storage. This function is equivalent to the expression LOG(X) / LOG(2).

  • LOG10(X)

    Returns the base-10 logarithm of X.

    mysql> SELECT LOG10(2);
            -> 0.30102999566398
    mysql> SELECT LOG10(100);
            -> 2
    mysql> SELECT LOG10(-100);
            -> NULL
    

    LOG10(X) is equivalent to LOG(10,X).

  • MOD(N,M), N % M, N MOD M

    Modulo operation. Returns the remainder of N divided by M.

    mysql> SELECT MOD(234, 10);
            -> 4
    mysql> SELECT 253 % 7;
            -> 1
    mysql> SELECT MOD(29,9);
            -> 2
    mysql> SELECT 29 MOD 9;
            -> 2
    

    This function is safe to use with BIGINT values.

    MOD() also works on values that have a fractional part and returns the exact remainder after division:

    mysql> SELECT MOD(34.5,3);
            -> 1.5
    

    MOD(N,0) returns NULL.

  • PI()

    Returns the value of π (pi). The default number of decimal places displayed is seven, but MySQL uses the full double-precision value internally.

    mysql> SELECT PI();
            -> 3.141593
    mysql> SELECT PI()+0.000000000000000000;
            -> 3.141592653589793116
    
  • POW(X,Y)

    Returns the value of X raised to the power of Y.

    mysql> SELECT POW(2,2);
            -> 4
    mysql> SELECT POW(2,-2);
            -> 0.25
    
  • POWER(X,Y)

    This is a synonym for POW().

  • RADIANS(X)

    Returns the argument X, converted from degrees to radians. (Note that π radians equals 180 degrees.)

    mysql> SELECT RADIANS(90);
            -> 1.5707963267949
    
  • RAND(), RAND(N)

    Returns a random floating-point value v in the range 0 <= v < 1.0. If a constant integer argument N is specified, it is used as the seed value, which produces a repeatable sequence of column values. In the following example, note that the sequences of values produced by RAND(3) is the same both places where it occurs.

    
    mysql> CREATE TABLE t (i INT);
    Query OK, 0 rows affected (0.42 sec)
    
    mysql> INSERT INTO t VALUES(1),(2),(3);
    Query OK, 3 rows affected (0.00 sec)
    Records: 3  Duplicates: 0  Warnings: 0
    
    mysql> SELECT i, RAND() FROM t;
    +------+------------------+
    | i    | RAND()           |
    +------+------------------+
    |    1 | 0.61914388706828 |
    |    2 | 0.93845168309142 |
    |    3 | 0.83482678498591 |
    +------+------------------+
    3 rows in set (0.00 sec)
    
    mysql> SELECT i, RAND(3) FROM t;
    +------+------------------+
    | i    | RAND(3)          |
    +------+------------------+
    |    1 | 0.90576975597606 |
    |    2 | 0.37307905813035 |
    |    3 | 0.14808605345719 |
    +------+------------------+
    3 rows in set (0.00 sec)
    
    mysql> SELECT i, RAND() FROM t;
    +------+------------------+
    | i    | RAND()           |
    +------+------------------+
    |    1 | 0.35877890638893 |
    |    2 | 0.28941420772058 |
    |    3 | 0.37073435016976 |
    +------+------------------+
    3 rows in set (0.00 sec)
    
    mysql> SELECT i, RAND(3) FROM t;
    +------+------------------+
    | i    | RAND(3)          |
    +------+------------------+
    |    1 | 0.90576975597606 |
    |    2 | 0.37307905813035 |
    |    3 | 0.14808605345719 |
    +------+------------------+
    3 rows in set (0.01 sec)
    

    With a constant initializer, the seed is initialized once when the statement is compiled, prior to execution. If a nonconstant initializer (such as a column name) is used as the argument, the seed is initialized with the value for each invocation of RAND(). (One implication of this is that for equal argument values, RAND() will return the same value each time.)

    To obtain a random integer R in the range i <= R < j, use the expression FLOOR(i + RAND() * (ji)). For example, to obtain a random integer in the range the range 7 <= R < 12, you could use the following statement:

    SELECT FLOOR(7 + (RAND() * 5));
    

    RAND() in a WHERE clause is re-evaluated every time the WHERE is executed.

    You cannot use a column with RAND() values in an ORDER BY clause, because ORDER BY would evaluate the column multiple times. However, you can retrieve rows in random order like this:

    mysql> SELECT * FROM tbl_name ORDER BY RAND();
    

    ORDER BY RAND() combined with LIMIT is useful for selecting a random sample from a set of rows:

    mysql> SELECT * FROM table1, table2 WHERE a=b AND c<d -> ORDER BY RAND() LIMIT 1000;
    

    RAND() is not meant to be a perfect random generator. It is a fast way to generate random numbers on demand that is portable between platforms for the same MySQL version.

    This function is unsafe for statement-based replication. Beginning with MySQL 5.5.2, a warning is logged if you use this function when binlog_format is set to STATEMENT. (Bug #49222)

  • ROUND(X), ROUND(X,D)

    Rounds the argument X to D decimal places. The rounding algorithm depends on the data type of X. D defaults to 0 if not specified. D can be negative to cause D digits left of the decimal point of the value X to become zero.

    mysql> SELECT ROUND(-1.23);
            -> -1
    mysql> SELECT ROUND(-1.58);
            -> -2
    mysql> SELECT ROUND(1.58);
            -> 2
    mysql> SELECT ROUND(1.298, 1);
            -> 1.3
    mysql> SELECT ROUND(1.298, 0);
            -> 1
    mysql> SELECT ROUND(23.298, -1);
            -> 20
    

    The return type is the same type as that of the first argument (assuming that it is integer, double, or decimal). This means that for an integer argument, the result is an integer (no decimal places):

    mysql> SELECT ROUND(150.000,2), ROUND(150,2);
    +------------------+--------------+
    | ROUND(150.000,2) | ROUND(150,2) |
    +------------------+--------------+
    |           150.00 |          150 |
    +------------------+--------------+
    

    ROUND() uses the following rules depending on the type of the first argument:

    • For exact-value numbers, ROUND() uses the round half away from zero or round toward nearest rule: A value with a fractional part of .5 or greater is rounded up to the next integer if positive or down to the next integer if negative. (In other words, it is rounded away from zero.) A value with a fractional part less than .5 is rounded down to the next integer if positive or up to the next integer if negative.

    • For approximate-value numbers, the result depends on the C library. On many systems, this means that ROUND() uses the "round to nearest even" rule: A value with any fractional part is rounded to the nearest even integer.

    The following example shows how rounding differs for exact and approximate values:

    mysql> SELECT ROUND(2.5), ROUND(25E-1);
    +------------+--------------+
    | ROUND(2.5) | ROUND(25E-1) |
    +------------+--------------+
    | 3          |            2 |
    +------------+--------------+
    

    For more information, see Section 12.18, “Precision Math”.

  • SIGN(X)

    Returns the sign of the argument as -1, 0, or 1, depending on whether X is negative, zero, or positive.

    mysql> SELECT SIGN(-32);
            -> -1
    mysql> SELECT SIGN(0);
            -> 0
    mysql> SELECT SIGN(234);
            -> 1
    
  • SIN(X)

    Returns the sine of X, where X is given in radians.

    mysql> SELECT SIN(PI());
            -> 1.2246063538224e-16
    mysql> SELECT ROUND(SIN(PI()));
            -> 0
    
  • SQRT(X)

    Returns the square root of a nonnegative number X.

    mysql> SELECT SQRT(4);
            -> 2
    mysql> SELECT SQRT(20);
            -> 4.4721359549996
    mysql> SELECT SQRT(-16);
            -> NULL
    
  • TAN(X)

    Returns the tangent of X, where X is given in radians.

    mysql> SELECT TAN(PI());
            -> -1.2246063538224e-16
    mysql> SELECT TAN(PI()+1);
            -> 1.5574077246549
    
  • TRUNCATE(X,D)

    Returns the number X, truncated to D decimal places. If D is 0, the result has no decimal point or fractional part. D can be negative to cause D digits left of the decimal point of the value X to become zero.

    mysql> SELECT TRUNCATE(1.223,1);
            -> 1.2
    mysql> SELECT TRUNCATE(1.999,1);
            -> 1.9
    mysql> SELECT TRUNCATE(1.999,0);
            -> 1
    mysql> SELECT TRUNCATE(-1.999,1);
            -> -1.9
    mysql> SELECT TRUNCATE(122,-2);
           -> 100
    mysql> SELECT TRUNCATE(10.28*100,0);
           -> 1028
    

    All numbers are rounded toward zero.

12.7 Date and Time Functions

This section describes the functions that can be used to manipulate temporal values. See Section 11.3, “Date and Time Types”, for a description of the range of values each date and time type has and the valid formats in which values may be specified.

Table 12.13 Date/Time Functions

NameDescription
ADDDATE()Add time values (intervals) to a date value
ADDTIME()Add time
CONVERT_TZ()Convert from one timezone to another
CURDATE()Return the current date
CURRENT_DATE(), CURRENT_DATESynonyms for CURDATE()
CURRENT_TIME(), CURRENT_TIMESynonyms for CURTIME()
CURRENT_TIMESTAMP(), CURRENT_TIMESTAMPSynonyms for NOW()
CURTIME()Return the current time
DATE_ADD()Add time values (intervals) to a date value
DATE_FORMAT()Format date as specified
DATE_SUB()Subtract a time value (interval) from a date
DATE()Extract the date part of a date or datetime expression
DATEDIFF()Subtract two dates
DAY()Synonym for DAYOFMONTH()
DAYNAME()Return the name of the weekday
DAYOFMONTH()Return the day of the month (0-31)
DAYOFWEEK()Return the weekday index of the argument
DAYOFYEAR()Return the day of the year (1-366)
EXTRACT()Extract part of a date
FROM_DAYS()Convert a day number to a date
FROM_UNIXTIME()Format UNIX timestamp as a date
GET_FORMAT()Return a date format string
HOUR()Extract the hour
LAST_DAYReturn the last day of the month for the argument
LOCALTIME(), LOCALTIMESynonym for NOW()
LOCALTIMESTAMP, LOCALTIMESTAMP()Synonym for NOW()
MAKEDATE()Create a date from the year and day of year
MAKETIME()Create time from hour, minute, second
MICROSECOND()Return the microseconds from argument
MINUTE()Return the minute from the argument
MONTH()Return the month from the date passed
MONTHNAME()Return the name of the month
NOW()Return the current date and time
PERIOD_ADD()Add a period to a year-month
PERIOD_DIFF()Return the number of months between periods
QUARTER()Return the quarter from a date argument
SEC_TO_TIME()Converts seconds to 'HH:MM:SS' format
SECOND()Return the second (0-59)
STR_TO_DATE()Convert a string to a date
SUBDATE()Synonym for DATE_SUB() when invoked with three arguments
SUBTIME()Subtract times
SYSDATE()Return the time at which the function executes
TIME_FORMAT()Format as time
TIME_TO_SEC()Return the argument converted to seconds
TIME()Extract the time portion of the expression passed
TIMEDIFF()Subtract time
TIMESTAMP()With a single argument, this function returns the date or datetime expression; with two arguments, the sum of the arguments
TIMESTAMPADD()Add an interval to a datetime expression
TIMESTAMPDIFF()Subtract an interval from a datetime expression
TO_DAYS()Return the date argument converted to days
TO_SECONDS()Return the date or datetime argument converted to seconds since Year 0
UNIX_TIMESTAMP()Return a UNIX timestamp
UTC_DATE()Return the current UTC date
UTC_TIME()Return the current UTC time
UTC_TIMESTAMP()Return the current UTC date and time
WEEK()Return the week number
WEEKDAY()Return the weekday index
WEEKOFYEAR()Return the calendar week of the date (0-53)
YEAR()Return the year
YEARWEEK()Return the year and week

Here is an example that uses date functions. The following query selects all rows with a date_col value from within the last 30 days:

mysql> SELECT something FROM tbl_name
    -> WHERE DATE_SUB(CURDATE(),INTERVAL 30 DAY) <= date_col;

The query also selects rows with dates that lie in the future.

Functions that expect date values usually accept datetime values and ignore the time part. Functions that expect time values usually accept datetime values and ignore the date part.

Functions that return the current date or time each are evaluated only once per query at the start of query execution. This means that multiple references to a function such as NOW() within a single query always produce the same result. (For our purposes, a single query also includes a call to a stored program (stored routine, trigger, or event) and all subprograms called by that program.) This principle also applies to CURDATE(), CURTIME(), UTC_DATE(), UTC_TIME(), UTC_TIMESTAMP(), and to any of their synonyms.

The CURRENT_TIMESTAMP(), CURRENT_TIME(), CURRENT_DATE(), and FROM_UNIXTIME() functions return values in the connection's current time zone, which is available as the value of the time_zone system variable. In addition, UNIX_TIMESTAMP() assumes that its argument is a datetime value in the current time zone. See Section 10.6, “MySQL Server Time Zone Support”.

Some date functions can be used with zero dates or incomplete dates such as '2001-11-00', whereas others cannot. Functions that extract parts of dates typically work with incomplete dates and thus can return 0 when you might otherwise expect a nonzero value. For example:

mysql> SELECT DAYOFMONTH('2001-11-00'), MONTH('2005-00-00');
        -> 0, 0

Other functions expect complete dates and return NULL for incomplete dates. These include functions that perform date arithmetic or that map parts of dates to names. For example:

mysql> SELECT DATE_ADD('2006-05-00',INTERVAL 1 DAY);
        -> NULL
mysql> SELECT DAYNAME('2006-05-00');
        -> NULL
Note

From MySQL 5.5.16 to 5.5.20, a change in handling of a date-related assertion caused several functions to become more strict when passed a DATE() function value as their argument and reject incomplete dates with a day part of zero. These functions are affected: CONVERT_TZ(), DATE_ADD(), DATE_SUB(), DAYOFYEAR(), LAST_DAY(), TIMESTAMPDIFF(), TO_DAYS(), TO_SECONDS(), WEEK(), WEEKDAY(), WEEKOFYEAR(), YEARWEEK(). Because this changes date-handling behavior in General Availability-status series MySQL 5.5, the change was reverted in 5.5.21.

  • ADDDATE(date,INTERVAL expr unit), ADDDATE(expr,days)

    When invoked with the INTERVAL form of the second argument, ADDDATE() is a synonym for DATE_ADD(). The related function SUBDATE() is a synonym for DATE_SUB(). For information on the INTERVAL unit argument, see the discussion for DATE_ADD().

    mysql> SELECT DATE_ADD('2008-01-02', INTERVAL 31 DAY);
            -> '2008-02-02'
    mysql> SELECT ADDDATE('2008-01-02', INTERVAL 31 DAY);
            -> '2008-02-02'
    

    When invoked with the days form of the second argument, MySQL treats it as an integer number of days to be added to expr.

    mysql> SELECT ADDDATE('2008-01-02', 31);
            -> '2008-02-02'
    
  • ADDTIME(expr1,expr2)

    ADDTIME() adds expr2 to expr1 and returns the result. expr1 is a time or datetime expression, and expr2 is a time expression.

    mysql> SELECT ADDTIME('2007-12-31 23:59:59.999999', '1 1:1:1.000002');
            -> '2008-01-02 01:01:01.000001'
    mysql> SELECT ADDTIME('01:00:00.999999', '02:00:00.999998');
            -> '03:00:01.999997'
    
  • CONVERT_TZ(dt,from_tz,to_tz)

    CONVERT_TZ() converts a datetime value dt from the time zone given by from_tz to the time zone given by to_tz and returns the resulting value. Time zones are specified as described in Section 10.6, “MySQL Server Time Zone Support”. This function returns NULL if the arguments are invalid.

    If the value falls out of the supported range of the TIMESTAMP type when converted from from_tz to UTC, no conversion occurs. The TIMESTAMP range is described in Section 11.1.2, “Date and Time Type Overview”.

    mysql> SELECT CONVERT_TZ('2004-01-01 12:00:00','GMT','MET');
            -> '2004-01-01 13:00:00'
    mysql> SELECT CONVERT_TZ('2004-01-01 12:00:00','+00:00','+10:00');
            -> '2004-01-01 22:00:00'
    
    Note

    To use named time zones such as 'MET' or 'Europe/Moscow', the time zone tables must be properly set up. See Section 10.6, “MySQL Server Time Zone Support”, for instructions.

  • CURDATE()

    Returns the current date as a value in 'YYYY-MM-DD' or YYYYMMDD format, depending on whether the function is used in a string or numeric context.

    mysql> SELECT CURDATE();
            -> '2008-06-13'
    mysql> SELECT CURDATE() + 0;
            -> 20080613
    
  • CURRENT_DATE, CURRENT_DATE()

    CURRENT_DATE and CURRENT_DATE() are synonyms for CURDATE().

  • CURRENT_TIME, CURRENT_TIME()

    CURRENT_TIME and CURRENT_TIME() are synonyms for CURTIME().

  • CURRENT_TIMESTAMP, CURRENT_TIMESTAMP()

    CURRENT_TIMESTAMP and CURRENT_TIMESTAMP() are synonyms for NOW().

  • CURTIME()

    Returns the current time as a value in 'HH:MM:SS' or HHMMSS.uuuuuu format, depending on whether the function is used in a string or numeric context. The value is expressed in the current time zone.

    mysql> SELECT CURTIME();
            -> '23:50:26'
    mysql> SELECT CURTIME() + 0;
            -> 235026.000000
    
  • DATE(expr)

    Extracts the date part of the date or datetime expression expr.

    mysql> SELECT DATE('2003-12-31 01:02:03');
            -> '2003-12-31'
    
  • DATEDIFF(expr1,expr2)

    DATEDIFF() returns expr1expr2 expressed as a value in days from one date to the other. expr1 and expr2 are date or date-and-time expressions. Only the date parts of the values are used in the calculation.

    mysql> SELECT DATEDIFF('2007-12-31 23:59:59','2007-12-30');
            -> 1
    mysql> SELECT DATEDIFF('2010-11-30 23:59:59','2010-12-31');
            -> -31
    
  • DATE_ADD(date,INTERVAL expr unit), DATE_SUB(date,INTERVAL expr unit)

    These functions perform date arithmetic. The date argument specifies the starting date or datetime value. expr is an expression specifying the interval value to be added or subtracted from the starting date. expr is a string; it may start with a - for negative intervals. unit is a keyword indicating the units in which the expression should be interpreted.

    The INTERVAL keyword and the unit specifier are not case sensitive.

    The following table shows the expected form of the expr argument for each unit value.

    unit ValueExpected expr Format
    MICROSECONDMICROSECONDS
    SECONDSECONDS
    MINUTEMINUTES
    HOURHOURS
    DAYDAYS
    WEEKWEEKS
    MONTHMONTHS
    QUARTERQUARTERS
    YEARYEARS
    SECOND_MICROSECOND'SECONDS.MICROSECONDS'
    MINUTE_MICROSECOND'MINUTES:SECONDS.MICROSECONDS'
    MINUTE_SECOND'MINUTES:SECONDS'
    HOUR_MICROSECOND'HOURS:MINUTES:SECONDS.MICROSECONDS'
    HOUR_SECOND'HOURS:MINUTES:SECONDS'
    HOUR_MINUTE'HOURS:MINUTES'
    DAY_MICROSECOND'DAYS HOURS:MINUTES:SECONDS.MICROSECONDS'
    DAY_SECOND'DAYS HOURS:MINUTES:SECONDS'
    DAY_MINUTE'DAYS HOURS:MINUTES'
    DAY_HOUR'DAYS HOURS'
    YEAR_MONTH'YEARS-MONTHS'

    The return value depends on the arguments:

    • DATETIME if the first argument is a DATETIME (or TIMESTAMP) value, or if the first argument is a DATE and the unit value uses HOURS, MINUTES, or SECONDS.

    • String otherwise.

    To ensure that the result is DATETIME, you can use CAST() to convert the first argument to DATETIME.

    MySQL permits any punctuation delimiter in the expr format. Those shown in the table are the suggested delimiters. If the date argument is a DATE value and your calculations involve only YEAR, MONTH, and DAY parts (that is, no time parts), the result is a DATE value. Otherwise, the result is a DATETIME value.

    Date arithmetic also can be performed using INTERVAL together with the + or - operator:

    date + INTERVAL expr unit
    date - INTERVAL expr unit
    

    INTERVAL expr unit is permitted on either side of the + operator if the expression on the other side is a date or datetime value. For the - operator, INTERVAL expr unit is permitted only on the right side, because it makes no sense to subtract a date or datetime value from an interval.

    mysql> SELECT '2008-12-31 23:59:59' + INTERVAL 1 SECOND;
            -> '2009-01-01 00:00:00'
    mysql> SELECT INTERVAL 1 DAY + '2008-12-31';
            -> '2009-01-01'
    mysql> SELECT '2005-01-01' - INTERVAL 1 SECOND;
            -> '2004-12-31 23:59:59'
    mysql> SELECT DATE_ADD('2000-12-31 23:59:59',
        ->                 INTERVAL 1 SECOND);
            -> '2001-01-01 00:00:00'
    mysql> SELECT DATE_ADD('2010-12-31 23:59:59',
        ->                 INTERVAL 1 DAY);
            -> '2011-01-01 23:59:59'
    mysql> SELECT DATE_ADD('2100-12-31 23:59:59',
        ->                 INTERVAL '1:1' MINUTE_SECOND);
            -> '2101-01-01 00:01:00'
    mysql> SELECT DATE_SUB('2005-01-01 00:00:00',
        ->                 INTERVAL '1 1:1:1' DAY_SECOND);
            -> '2004-12-30 22:58:59'
    mysql> SELECT DATE_ADD('1900-01-01 00:00:00',
        ->                 INTERVAL '-1 10' DAY_HOUR);
            -> '1899-12-30 14:00:00'
    mysql> SELECT DATE_SUB('1998-01-02', INTERVAL 31 DAY);
            -> '1997-12-02'
    mysql> SELECT DATE_ADD('1992-12-31 23:59:59.000002',
        ->            INTERVAL '1.999999' SECOND_MICROSECOND);
            -> '1993-01-01 00:00:01.000001'
    

    If you specify an interval value that is too short (does not include all the interval parts that would be expected from the unit keyword), MySQL assumes that you have left out the leftmost parts of the interval value. For example, if you specify a unit of DAY_SECOND, the value of expr is expected to have days, hours, minutes, and seconds parts. If you specify a value like '1:10', MySQL assumes that the days and hours parts are missing and the value represents minutes and seconds. In other words, '1:10' DAY_SECOND is interpreted in such a way that it is equivalent to '1:10' MINUTE_SECOND. This is analogous to the way that MySQL interprets TIME values as representing elapsed time rather than as a time of day.

    Because expr is treated as a string, be careful if you specify a nonstring value with INTERVAL. For example, with an interval specifier of HOUR_MINUTE, 6/4 evaluates to 1.5000 and is treated as 1 hour, 5000 minutes:

    mysql> SELECT 6/4;
            -> 1.5000
    mysql> SELECT DATE_ADD('2009-01-01', INTERVAL 6/4 HOUR_MINUTE);
            -> '2009-01-04 12:20:00'
    

    To ensure interpretation of the interval value as you expect, a CAST() operation may be used. To treat 6/4 as 1 hour, 5 minutes, cast it to a DECIMAL value with a single fractional digit:

    mysql> SELECT CAST(6/4 AS DECIMAL(3,1));
            -> 1.5
    mysql> SELECT DATE_ADD('1970-01-01 12:00:00',
        ->                 INTERVAL CAST(6/4 AS DECIMAL(3,1)) HOUR_MINUTE);
            -> '1970-01-01 13:05:00'
    

    If you add to or subtract from a date value something that contains a time part, the result is automatically converted to a datetime value:

    mysql> SELECT DATE_ADD('2013-01-01', INTERVAL 1 DAY);
            -> '2013-01-02'
    mysql> SELECT DATE_ADD('2013-01-01', INTERVAL 1 HOUR);
            -> '2013-01-01 01:00:00'
    

    If you add MONTH, YEAR_MONTH, or YEAR and the resulting date has a day that is larger than the maximum day for the new month, the day is adjusted to the maximum days in the new month:

    mysql> SELECT DATE_ADD('2009-01-30', INTERVAL 1 MONTH);
            -> '2009-02-28'
    

    Date arithmetic operations require complete dates and do not work with incomplete dates such as '2006-07-00' or badly malformed dates:

    mysql> SELECT DATE_ADD('2006-07-00', INTERVAL 1 DAY);
            -> NULL
    mysql> SELECT '2005-03-32' + INTERVAL 1 MONTH;
            -> NULL
    
  • DATE_FORMAT(date,format)

    Formats the date value according to the format string.

    The following specifiers may be used in the format string. The % character is required before format specifier characters.

    SpecifierDescription
    %aAbbreviated weekday name (Sun..Sat)
    %bAbbreviated month name (Jan..Dec)
    %cMonth, numeric (0..12)
    %DDay of the month with English suffix (0th, 1st, 2nd, 3rd, …)
    %dDay of the month, numeric (00..31)
    %eDay of the month, numeric (0..31)
    %fMicroseconds (000000..999999)
    %HHour (00..23)
    %hHour (01..12)
    %IHour (01..12)
    %iMinutes, numeric (00..59)
    %jDay of year (001..366)
    %kHour (0..23)
    %lHour (1..12)
    %MMonth name (January..December)
    %mMonth, numeric (00..12)
    %pAM or PM
    %rTime, 12-hour (hh:mm:ss followed by AM or PM)
    %SSeconds (00..59)
    %sSeconds (00..59)
    %TTime, 24-hour (hh:mm:ss)
    %UWeek (00..53), where Sunday is the first day of the week; WEEK() mode 0
    %uWeek (00..53), where Monday is the first day of the week; WEEK() mode 1
    %VWeek (01..53), where Sunday is the first day of the week; WEEK() mode 2; used with %X
    %vWeek (01..53), where Monday is the first day of the week; WEEK() mode 3; used with %x
    %WWeekday name (Sunday..Saturday)
    %wDay of the week (0=Sunday..6=Saturday)
    %XYear for the week where Sunday is the first day of the week, numeric, four digits; used with %V
    %xYear for the week, where Monday is the first day of the week, numeric, four digits; used with %v
    %YYear, numeric, four digits
    %yYear, numeric (two digits)
    %%A literal % character
    %xx, for any x not listed above

    Ranges for the month and day specifiers begin with zero due to the fact that MySQL permits the storing of incomplete dates such as '2014-00-00'.

    The language used for day and month names and abbreviations is controlled by the value of the lc_time_names system variable (Section 10.7, “MySQL Server Locale Support”).

    For the %U, %u, %V, and %v specifiers, see the description of the WEEK() function for information about the mode values. The mode affects how week numbering occurs.

    DATE_FORMAT() returns a string with a character set and collation given by character_set_connection and collation_connection so that it can return month and weekday names containing non-ASCII characters.

    mysql> SELECT DATE_FORMAT('2009-10-04 22:23:00', '%W %M %Y');
            -> 'Sunday October 2009'
    mysql> SELECT DATE_FORMAT('2007-10-04 22:23:00', '%H:%i:%s');
            -> '22:23:00'
    mysql> SELECT DATE_FORMAT('1900-10-04 22:23:00',
        ->                 '%D %y %a %d %m %b %j');
            -> '4th 00 Thu 04 10 Oct 277'
    mysql> SELECT DATE_FORMAT('1997-10-04 22:23:00',
        ->                 '%H %k %I %r %T %S %w');
            -> '22 22 10 10:23:00 PM 22:23:00 00 6'
    mysql> SELECT DATE_FORMAT('1999-01-01', '%X %V');
            -> '1998 52'
    mysql> SELECT DATE_FORMAT('2006-06-00', '%d');
            -> '00'
    
  • DATE_SUB(date,INTERVAL expr unit)

    See the description for DATE_ADD().

  • DAY(date)

    DAY() is a synonym for DAYOFMONTH().

  • DAYNAME(date)

    Returns the name of the weekday for date. The language used for the name is controlled by the value of the lc_time_names system variable (Section 10.7, “MySQL Server Locale Support”).

    mysql> SELECT DAYNAME('2007-02-03');
            -> 'Saturday'
    
  • DAYOFMONTH(date)

    Returns the day of the month for date, in the range 1 to 31, or 0 for dates such as '0000-00-00' or '2008-00-00' that have a zero day part.

    mysql> SELECT DAYOFMONTH('2007-02-03');
            -> 3
    
  • DAYOFWEEK(date)

    Returns the weekday index for date (1 = Sunday, 2 = Monday, …, 7 = Saturday). These index values correspond to the ODBC standard.

    mysql> SELECT DAYOFWEEK('2007-02-03');
            -> 7
    
  • DAYOFYEAR(date)

    Returns the day of the year for date, in the range 1 to 366.

    mysql> SELECT DAYOFYEAR('2007-02-03');
            -> 34
    
  • EXTRACT(unit FROM date)

    The EXTRACT() function uses the same kinds of unit specifiers as DATE_ADD() or DATE_SUB(), but extracts parts from the date rather than performing date arithmetic.

    mysql> SELECT EXTRACT(YEAR FROM '2009-07-02');
           -> 2009
    mysql> SELECT EXTRACT(YEAR_MONTH FROM '2009-07-02 01:02:03');
           -> 200907
    mysql> SELECT EXTRACT(DAY_MINUTE FROM '2009-07-02 01:02:03');
           -> 20102
    mysql> SELECT EXTRACT(MICROSECOND
        ->                FROM '2003-01-02 10:30:00.000123');
            -> 123
    
  • FROM_DAYS(N)

    Given a day number N, returns a DATE value.

    mysql> SELECT FROM_DAYS(730669);
            -> '2007-07-03'
    

    Use FROM_DAYS() with caution on old dates. It is not intended for use with values that precede the advent of the Gregorian calendar (1582). See Section 12.8, “What Calendar Is Used By MySQL?”.

  • FROM_UNIXTIME(unix_timestamp), FROM_UNIXTIME(unix_timestamp,format)

    Returns a representation of the unix_timestamp argument as a value in 'YYYY-MM-DD HH:MM:SS' or YYYYMMDDHHMMSS.uuuuuu format, depending on whether the function is used in a string or numeric context. The value is expressed in the current time zone. unix_timestamp is an internal timestamp value such as is produced by the UNIX_TIMESTAMP() function.

    If format is given, the result is formatted according to the format string, which is used the same way as listed in the entry for the DATE_FORMAT() function.

    mysql> SELECT FROM_UNIXTIME(1196440219);
            -> '2007-11-30 10:30:19'
    mysql> SELECT FROM_UNIXTIME(1196440219) + 0;
            -> 20071130103019.000000
    mysql> SELECT FROM_UNIXTIME(UNIX_TIMESTAMP(),
        ->                      '%Y %D %M %h:%i:%s %x');
            -> '2007 30th November 10:30:59 2007'
    

    Note: If you use UNIX_TIMESTAMP() and FROM_UNIXTIME() to convert between TIMESTAMP values and Unix timestamp values, the conversion is lossy because the mapping is not one-to-one in both directions. For details, see the description of the UNIX_TIMESTAMP() function.

  • GET_FORMAT({DATE|TIME|DATETIME}, {'EUR'|'USA'|'JIS'|'ISO'|'INTERNAL'})

    Returns a format string. This function is useful in combination with the DATE_FORMAT() and the STR_TO_DATE() functions.

    The possible values for the first and second arguments result in several possible format strings (for the specifiers used, see the table in the DATE_FORMAT() function description). ISO format refers to ISO 9075, not ISO 8601.

    TIMESTAMP can also be used as the first argument to GET_FORMAT(), in which case the function returns the same values as for DATETIME.

    mysql> SELECT DATE_FORMAT('2003-10-03',GET_FORMAT(DATE,'EUR'));
            -> '03.10.2003'
    mysql> SELECT STR_TO_DATE('10.31.2003',GET_FORMAT(DATE,'USA'));
            -> '2003-10-31'
    
  • HOUR(time)

    Returns the hour for time. The range of the return value is 0 to 23 for time-of-day values. However, the range of TIME values actually is much larger, so HOUR can return values greater than 23.

    mysql> SELECT HOUR('10:05:03');
            -> 10
    mysql> SELECT HOUR('272:59:59');
            -> 272
    
  • LAST_DAY(date)

    Takes a date or datetime value and returns the corresponding value for the last day of the month. Returns NULL if the argument is invalid.

    mysql> SELECT LAST_DAY('2003-02-05');
            -> '2003-02-28'
    mysql> SELECT LAST_DAY('2004-02-05');
            -> '2004-02-29'
    mysql> SELECT LAST_DAY('2004-01-01 01:01:01');
            -> '2004-01-31'
    mysql> SELECT LAST_DAY('2003-03-32');
            -> NULL
    
  • LOCALTIME, LOCALTIME()

    LOCALTIME and LOCALTIME() are synonyms for NOW().

  • LOCALTIMESTAMP, LOCALTIMESTAMP()

    LOCALTIMESTAMP and LOCALTIMESTAMP() are synonyms for NOW().

  • MAKEDATE(year,dayofyear)

    Returns a date, given year and day-of-year values. dayofyear must be greater than 0 or the result is NULL.

    mysql> SELECT MAKEDATE(2011,31), MAKEDATE(2011,32);
            -> '2011-01-31', '2011-02-01'
    mysql> SELECT MAKEDATE(2011,365), MAKEDATE(2014,365);
            -> '2011-12-31', '2014-12-31'
    mysql> SELECT MAKEDATE(2011,0);
            -> NULL
    
  • MAKETIME(hour,minute,second)

    Returns a time value calculated from the hour, minute, and second arguments.

    mysql> SELECT MAKETIME(12,15,30);
            -> '12:15:30'
    
  • MICROSECOND(expr)

    Returns the microseconds from the time or datetime expression expr as a number in the range from 0 to 999999.

    mysql> SELECT MICROSECOND('12:00:00.123456');
            -> 123456
    mysql> SELECT MICROSECOND('2009-12-31 23:59:59.000010');
            -> 10
    
  • MINUTE(time)

    Returns the minute for time, in the range 0 to 59.

    mysql> SELECT MINUTE('2008-02-03 10:05:03');
            -> 5
    
  • MONTH(date)

    Returns the month for date, in the range 1 to 12 for January to December, or 0 for dates such as '0000-00-00' or '2008-00-00' that have a zero month part.

    mysql> SELECT MONTH('2008-02-03');
            -> 2
    
  • MONTHNAME(date)

    Returns the full name of the month for date. The language used for the name is controlled by the value of the lc_time_names system variable (Section 10.7, “MySQL Server Locale Support”).

    mysql> SELECT MONTHNAME('2008-02-03');
            -> 'February'
    
  • NOW()

    Returns the current date and time as a value in 'YYYY-MM-DD HH:MM:SS' or YYYYMMDDHHMMSS.uuuuuu format, depending on whether the function is used in a string or numeric context. The value is expressed in the current time zone.

    mysql> SELECT NOW();
            -> '2007-12-15 23:50:26'
    mysql> SELECT NOW() + 0;
            -> 20071215235026.000000
    

    NOW() returns a constant time that indicates the time at which the statement began to execute. (Within a stored function or trigger, NOW() returns the time at which the function or triggering statement began to execute.) This differs from the behavior for SYSDATE(), which returns the exact time at which it executes.

    mysql> SELECT NOW(), SLEEP(2), NOW();
    +---------------------+----------+---------------------+
    | NOW()               | SLEEP(2) | NOW()               |
    +---------------------+----------+---------------------+
    | 2006-04-12 13:47:36 |        0 | 2006-04-12 13:47:36 |
    +---------------------+----------+---------------------+
    
    mysql> SELECT SYSDATE(), SLEEP(2), SYSDATE();
    +---------------------+----------+---------------------+
    | SYSDATE()           | SLEEP(2) | SYSDATE()           |
    +---------------------+----------+---------------------+
    | 2006-04-12 13:47:44 |        0 | 2006-04-12 13:47:46 |
    +---------------------+----------+---------------------+
    

    In addition, the SET TIMESTAMP statement affects the value returned by NOW() but not by SYSDATE(). This means that timestamp settings in the binary log have no effect on invocations of SYSDATE(). Setting the timestamp to a nonzero value causes each subsequent invocation of NOW() to return that value. Setting the timestamp to zero cancels this effect so that NOW() once again returns the current date and time.

    See the description for SYSDATE() for additional information about the differences between the two functions.

  • PERIOD_ADD(P,N)

    Adds N months to period P (in the format YYMM or YYYYMM). Returns a value in the format YYYYMM. Note that the period argument P is not a date value.

    mysql> SELECT PERIOD_ADD(200801,2);
            -> 200803
    
  • PERIOD_DIFF(P1,P2)

    Returns the number of months between periods P1 and P2. P1 and P2 should be in the format YYMM or YYYYMM. Note that the period arguments P1 and P2 are not date values.

    mysql> SELECT PERIOD_DIFF(200802,200703);
            -> 11
    
  • QUARTER(date)

    Returns the quarter of the year for date, in the range 1 to 4.

    mysql> SELECT QUARTER('2008-04-01');
            -> 2
    
  • SECOND(time)

    Returns the second for time, in the range 0 to 59.

    mysql> SELECT SECOND('10:05:03');
            -> 3
    
  • SEC_TO_TIME(seconds)

    Returns the seconds argument, converted to hours, minutes, and seconds, as a TIME value. The range of the result is constrained to that of the TIME data type. A warning occurs if the argument corresponds to a value outside that range.

    mysql> SELECT SEC_TO_TIME(2378);
            -> '00:39:38'
    mysql> SELECT SEC_TO_TIME(2378) + 0;
            -> 3938
    
  • STR_TO_DATE(str,format)

    This is the inverse of the DATE_FORMAT() function. It takes a string str and a format string format. STR_TO_DATE() returns a DATETIME value if the format string contains both date and time parts, or a DATE or TIME value if the string contains only date or time parts. If the date, time, or datetime value extracted from str is illegal, STR_TO_DATE() returns NULL and produces a warning.

    The server scans str attempting to match format to it. The format string can contain literal characters and format specifiers beginning with %. Literal characters in format must match literally in str. Format specifiers in format must match a date or time part in str. For the specifiers that can be used in format, see the DATE_FORMAT() function description.

    mysql> SELECT STR_TO_DATE('01,5,2013','%d,%m,%Y');
            -> '2013-05-01'
    mysql> SELECT STR_TO_DATE('May 1, 2013','%M %d,%Y');
            -> '2013-05-01'
    

    Scanning starts at the beginning of str and fails if format is found not to match. Extra characters at the end of str are ignored.

    mysql> SELECT STR_TO_DATE('a09:30:17','a%h:%i:%s');
            -> '09:30:17'
    mysql> SELECT STR_TO_DATE('a09:30:17','%h:%i:%s');
            -> NULL
    mysql> SELECT STR_TO_DATE('09:30:17a','%h:%i:%s');
            -> '09:30:17'
    

    Unspecified date or time parts have a value of 0, so incompletely specified values in str produce a result with some or all parts set to 0:

    mysql> SELECT STR_TO_DATE('abc','abc');
            -> '0000-00-00'
    mysql> SELECT STR_TO_DATE('9','%m');
            -> '0000-09-00'
    mysql> SELECT STR_TO_DATE('9','%s');
            -> '00:00:09'
    

    Range checking on the parts of date values is as described in Section 11.3.1, “The DATE, DATETIME, and TIMESTAMP Types”. This means, for example, that zero dates or dates with part values of 0 are permitted unless the SQL mode is set to disallow such values.

    mysql> SELECT STR_TO_DATE('00/00/0000', '%m/%d/%Y');
            -> '0000-00-00'
    mysql> SELECT STR_TO_DATE('04/31/2004', '%m/%d/%Y');
            -> '2004-04-31'
    
    Note

    You cannot use format "%X%V" to convert a year-week string to a date because the combination of a year and week does not uniquely identify a year and month if the week crosses a month boundary. To convert a year-week to a date, you should also specify the weekday:

    mysql> SELECT STR_TO_DATE('200442 Monday', '%X%V %W');
            -> '2004-10-18'
    
  • SUBDATE(date,INTERVAL expr unit), SUBDATE(expr,days)

    When invoked with the INTERVAL form of the second argument, SUBDATE() is a synonym for DATE_SUB(). For information on the INTERVAL unit argument, see the discussion for DATE_ADD().

    mysql> SELECT DATE_SUB('2008-01-02', INTERVAL 31 DAY);
            -> '2007-12-02'
    mysql> SELECT SUBDATE('2008-01-02', INTERVAL 31 DAY);
            -> '2007-12-02'
    

    The second form enables the use of an integer value for days. In such cases, it is interpreted as the number of days to be subtracted from the date or datetime expression expr.

    mysql> SELECT SUBDATE('2008-01-02 12:00:00', 31);
            -> '2007-12-02 12:00:00'
    
  • SUBTIME(expr1,expr2)

    SUBTIME() returns expr1expr2 expressed as a value in the same format as expr1. expr1 is a time or datetime expression, and expr2 is a time expression.

    mysql> SELECT SUBTIME('2007-12-31 23:59:59.999999','1 1:1:1.000002');
            -> '2007-12-30 22:58:58.999997'
    mysql> SELECT SUBTIME('01:00:00.999999', '02:00:00.999998');
            -> '-00:59:59.999999'
    
  • SYSDATE()

    Returns the current date and time as a value in 'YYYY-MM-DD HH:MM:SS' or YYYYMMDDHHMMSS.uuuuuu format, depending on whether the function is used in a string or numeric context.

    SYSDATE() returns the time at which it executes. This differs from the behavior for NOW(), which returns a constant time that indicates the time at which the statement began to execute. (Within a stored function or trigger, NOW() returns the time at which the function or triggering statement began to execute.)

    mysql> SELECT NOW(), SLEEP(2), NOW();
    +---------------------+----------+---------------------+
    | NOW()               | SLEEP(2) | NOW()               |
    +---------------------+----------+---------------------+
    | 2006-04-12 13:47:36 |        0 | 2006-04-12 13:47:36 |
    +---------------------+----------+---------------------+
    
    mysql> SELECT SYSDATE(), SLEEP(2), SYSDATE();
    +---------------------+----------+---------------------+
    | SYSDATE()           | SLEEP(2) | SYSDATE()           |
    +---------------------+----------+---------------------+
    | 2006-04-12 13:47:44 |        0 | 2006-04-12 13:47:46 |
    +---------------------+----------+---------------------+
    

    In addition, the SET TIMESTAMP statement affects the value returned by NOW() but not by SYSDATE(). This means that timestamp settings in the binary log have no effect on invocations of SYSDATE().

    Because SYSDATE() can return different values even within the same statement, and is not affected by SET TIMESTAMP, it is nondeterministic and therefore unsafe for replication if statement-based binary logging is used. If that is a problem, you can use row-based logging.

    Alternatively, you can use the --sysdate-is-now option to cause SYSDATE() to be an alias for NOW(). This works if the option is used on both the master and the slave.

    The nondeterministic nature of SYSDATE() also means that indexes cannot be used for evaluating expressions that refer to it.

  • TIME(expr)

    Extracts the time part of the time or datetime expression expr and returns it as a string.

    This function is unsafe for statement-based replication. Beginning with MySQL 5.5.1, a warning is logged if you use this function when binlog_format is set to STATEMENT. (Bug #47995)

    mysql> SELECT TIME('2003-12-31 01:02:03');
            -> '01:02:03'
    mysql> SELECT TIME('2003-12-31 01:02:03.000123');
            -> '01:02:03.000123'
    
  • TIMEDIFF(expr1,expr2)

    TIMEDIFF() returns expr1expr2 expressed as a time value. expr1 and expr2 are time or date-and-time expressions, but both must be of the same type.

    The result returned by TIMEDIFF() is limited to the range allowed for TIME values. Alternatively, you can use either of the functions TIMESTAMPDIFF() and UNIX_TIMESTAMP(), both of which return integers.

    mysql> SELECT TIMEDIFF('2000:01:01 00:00:00',
        ->                 '2000:01:01 00:00:00.000001');
            -> '-00:00:00.000001'
    mysql> SELECT TIMEDIFF('2008-12-31 23:59:59.000001',
        ->                 '2008-12-30 01:01:01.000002');
            -> '46:58:57.999999'
    
  • TIMESTAMP(expr), TIMESTAMP(expr1,expr2)

    With a single argument, this function returns the date or datetime expression expr as a datetime value. With two arguments, it adds the time expression expr2 to the date or datetime expression expr1 and returns the result as a datetime value.

    mysql> SELECT TIMESTAMP('2003-12-31');
            -> '2003-12-31 00:00:00'
    mysql> SELECT TIMESTAMP('2003-12-31 12:00:00','12:00:00');
            -> '2004-01-01 00:00:00'
    
  • TIMESTAMPADD(unit,interval,datetime_expr)

    Adds the integer expression interval to the date or datetime expression datetime_expr. The unit for interval is given by the unit argument, which should be one of the following values: MICROSECOND (microseconds), SECOND, MINUTE, HOUR, DAY, WEEK, MONTH, QUARTER, or YEAR.

    It is possible to use FRAC_SECOND in place of MICROSECOND, but FRAC_SECOND is deprecated. FRAC_SECOND was removed in MySQL 5.5.3.

    The unit value may be specified using one of keywords as shown, or with a prefix of SQL_TSI_. For example, DAY and SQL_TSI_DAY both are legal.

    mysql> SELECT TIMESTAMPADD(MINUTE,1,'2003-01-02');
            -> '2003-01-02 00:01:00'
    mysql> SELECT TIMESTAMPADD(WEEK,1,'2003-01-02');
            -> '2003-01-09'
    
  • TIMESTAMPDIFF(unit,datetime_expr1,datetime_expr2)

    Returns datetime_expr2datetime_expr1, where datetime_expr1 and datetime_expr2 are date or datetime expressions. One expression may be a date and the other a datetime; a date value is treated as a datetime having the time part '00:00:00' where necessary. The unit for the result (an integer) is given by the unit argument. The legal values for unit are the same as those listed in the description of the TIMESTAMPADD() function.

    mysql> SELECT TIMESTAMPDIFF(MONTH,'2003-02-01','2003-05-01');
            -> 3
    mysql> SELECT TIMESTAMPDIFF(YEAR,'2002-05-01','2001-01-01');
            -> -1
    mysql> SELECT TIMESTAMPDIFF(MINUTE,'2003-02-01','2003-05-01 12:05:55');
            -> 128885
    
    Note

    The order of the date or datetime arguments for this function is the opposite of that used with the TIMESTAMP() function when invoked with 2 arguments.

  • TIME_FORMAT(time,format)

    This is used like the DATE_FORMAT() function, but the format string may contain format specifiers only for hours, minutes, seconds, and microseconds. Other specifiers produce a NULL value or 0.

    If the time value contains an hour part that is greater than 23, the %H and %k hour format specifiers produce a value larger than the usual range of 0..23. The other hour format specifiers produce the hour value modulo 12.

    mysql> SELECT TIME_FORMAT('100:00:00', '%H %k %h %I %l');
            -> '100 100 04 04 4'
    
  • TIME_TO_SEC(time)

    Returns the time argument, converted to seconds.

    mysql> SELECT TIME_TO_SEC('22:23:00');
            -> 80580
    mysql> SELECT TIME_TO_SEC('00:39:38');
            -> 2378
    
  • TO_DAYS(date)

    Given a date date, returns a day number (the number of days since year 0).

    mysql> SELECT TO_DAYS(950501);
            -> 728779
    mysql> SELECT TO_DAYS('2007-10-07');
            -> 733321
    

    TO_DAYS() is not intended for use with values that precede the advent of the Gregorian calendar (1582), because it does not take into account the days that were lost when the calendar was changed. For dates before 1582 (and possibly a later year in other locales), results from this function are not reliable. See Section 12.8, “What Calendar Is Used By MySQL?”, for details.

    Remember that MySQL converts two-digit year values in dates to four-digit form using the rules in Section 11.3, “Date and Time Types”. For example, '2008-10-07' and '08-10-07' are seen as identical dates:

    mysql> SELECT TO_DAYS('2008-10-07'), TO_DAYS('08-10-07');
            -> 733687, 733687
    

    In MySQL, the zero date is defined as '0000-00-00', even though this date is itself considered invalid. This means that, for '0000-00-00' and '0000-01-01', TO_DAYS() returns the values shown here:

    mysql> SELECT TO_DAYS('0000-00-00');
    +-----------------------+
    | to_days('0000-00-00') |
    +-----------------------+
    |                  NULL |
    +-----------------------+
    1 row in set, 1 warning (0.00 sec)
    
    mysql> SHOW WARNINGS;
    +---------+------+----------------------------------------+
    | Level   | Code | Message                                |
    +---------+------+----------------------------------------+
    | Warning | 1292 | Incorrect datetime value: '0000-00-00' |
    +---------+------+----------------------------------------+
    1 row in set (0.00 sec)
    
    
    mysql> SELECT TO_DAYS('0000-01-01');
    +-----------------------+
    | to_days('0000-01-01') |
    +-----------------------+
    |                     1 |
    +-----------------------+
    1 row in set (0.00 sec)
    

    This is true whether or not the ALLOW_INVALID_DATES SQL server mode is enabled.

  • TO_SECONDS(expr)

    Given a date or datetime expr, returns a the number of seconds since the year 0. If expr is not a valid date or datetime value, returns NULL.

    mysql> SELECT TO_SECONDS(950501);
            -> 62966505600
    mysql> SELECT TO_SECONDS('2009-11-29');
            -> 63426672000
    mysql> SELECT TO_SECONDS('2009-11-29 13:43:32');
            -> 63426721412
    mysql> SELECT TO_SECONDS( NOW() );
            -> 63426721458
    

    Like TO_DAYS(), TO_SECONDS() is not intended for use with values that precede the advent of the Gregorian calendar (1582), because it does not take into account the days that were lost when the calendar was changed. For dates before 1582 (and possibly a later year in other locales), results from this function are not reliable. See Section 12.8, “What Calendar Is Used By MySQL?”, for details.

    Like TO_DAYS(), TO_SECONDS(), converts two-digit year values in dates to four-digit form using the rules in Section 11.3, “Date and Time Types”.

    TO_SECONDS() is available beginning with MySQL 5.5.0.

    In MySQL, the zero date is defined as '0000-00-00', even though this date is itself considered invalid. This means that, for '0000-00-00' and '0000-01-01', TO_SECONDS() returns the values shown here:

    mysql> SELECT TO_SECONDS('0000-00-00');
    +--------------------------+
    | TO_SECONDS('0000-00-00') |
    +--------------------------+
    |                     NULL |
    +--------------------------+
    1 row in set, 1 warning (0.00 sec)
    
    mysql> SHOW WARNINGS;
    +---------+------+----------------------------------------+
    | Level   | Code | Message                                |
    +---------+------+----------------------------------------+
    | Warning | 1292 | Incorrect datetime value: '0000-00-00' |
    +---------+------+----------------------------------------+
    1 row in set (0.00 sec)
    
    
    mysql> SELECT TO_SECONDS('0000-01-01');
    +--------------------------+
    | TO_SECONDS('0000-01-01') |
    +--------------------------+
    |                    86400 |
    +--------------------------+
    1 row in set (0.00 sec)
    

    This is true whether or not the ALLOW_INVALID_DATES SQL server mode is enabled.

  • UNIX_TIMESTAMP(), UNIX_TIMESTAMP(date)

    If called with no argument, returns a Unix timestamp (seconds since '1970-01-01 00:00:00' UTC) as an unsigned integer. If UNIX_TIMESTAMP() is called with a date argument, it returns the value of the argument as seconds since '1970-01-01 00:00:00' UTC. date may be a DATE string, a DATETIME string, a TIMESTAMP, or a number in the format YYMMDD or YYYYMMDD. The server interprets date as a value in the current time zone and converts it to an internal value in UTC. Clients can set their time zone as described in Section 10.6, “MySQL Server Time Zone Support”.

    mysql> SELECT UNIX_TIMESTAMP();
            -> 1196440210
    mysql> SELECT UNIX_TIMESTAMP('2007-11-30 10:30:19');
            -> 1196440219
    

    When UNIX_TIMESTAMP() is used on a TIMESTAMP column, the function returns the internal timestamp value directly, with no implicit string-to-Unix-timestamp conversion. If you pass an out-of-range date to UNIX_TIMESTAMP(), it returns 0.

    Note: If you use UNIX_TIMESTAMP() and FROM_UNIXTIME() to convert between TIMESTAMP values and Unix timestamp values, the conversion is lossy because the mapping is not one-to-one in both directions. For example, due to conventions for local time zone changes, it is possible for two UNIX_TIMESTAMP() to map two TIMESTAMP values to the same Unix timestamp value. FROM_UNIXTIME() will map that value back to only one of the original TIMESTAMP values. Here is an example, using TIMESTAMP values in the CET time zone:

    
    mysql> SELECT UNIX_TIMESTAMP('2005-03-27 03:00:00');
    +---------------------------------------+
    | UNIX_TIMESTAMP('2005-03-27 03:00:00') |
    +---------------------------------------+
    |                            1111885200 |
    +---------------------------------------+
    mysql> SELECT UNIX_TIMESTAMP('2005-03-27 02:00:00');
    +---------------------------------------+
    | UNIX_TIMESTAMP('2005-03-27 02:00:00') |
    +---------------------------------------+
    |                            1111885200 |
    +---------------------------------------+
    mysql> SELECT FROM_UNIXTIME(1111885200);
    +---------------------------+
    | FROM_UNIXTIME(1111885200) |
    +---------------------------+
    | 2005-03-27 03:00:00       |
    +---------------------------+
    

    If you want to subtract UNIX_TIMESTAMP() columns, you might want to cast the result to signed integers. See Section 12.10, “Cast Functions and Operators”.

  • UTC_DATE, UTC_DATE()

    Returns the current UTC date as a value in 'YYYY-MM-DD' or YYYYMMDD format, depending on whether the function is used in a string or numeric context.

    mysql> SELECT UTC_DATE(), UTC_DATE() + 0;
            -> '2003-08-14', 20030814
    
  • UTC_TIME, UTC_TIME()

    Returns the current UTC time as a value in 'HH:MM:SS' or HHMMSS.uuuuuu format, depending on whether the function is used in a string or numeric context.

    mysql> SELECT UTC_TIME(), UTC_TIME() + 0;
            -> '18:07:53', 180753.000000
    
  • UTC_TIMESTAMP, UTC_TIMESTAMP()

    Returns the current UTC date and time as a value in 'YYYY-MM-DD HH:MM:SS' or YYYYMMDDHHMMSS.uuuuuu format, depending on whether the function is used in a string or numeric context.

    mysql> SELECT UTC_TIMESTAMP(), UTC_TIMESTAMP() + 0;
            -> '2003-08-14 18:08:04', 20030814180804.000000
    
  • WEEK(date[,mode])

    This function returns the week number for date. The two-argument form of WEEK() enables you to specify whether the week starts on Sunday or Monday and whether the return value should be in the range from 0 to 53 or from 1 to 53. If the mode argument is omitted, the value of the default_week_format system variable is used. See Section 5.1.4, “Server System Variables”.

    The following table describes how the mode argument works.

    ModeFirst day of weekRangeWeek 1 is the first week …
    0Sunday0-53with a Sunday in this year
    1Monday0-53with 4 or more days this year
    2Sunday1-53with a Sunday in this year
    3Monday1-53with 4 or more days this year
    4Sunday0-53with 4 or more days this year
    5Monday0-53with a Monday in this year
    6Sunday1-53with 4 or more days this year
    7Monday1-53with a Monday in this year

    For mode values with a meaning of with 4 or more days this year, weeks are numbered according to ISO 8601:1988:

    • If the week containing January 1 has 4 or more days in the new year, it is week 1.

    • Otherwise, it is the last week of the previous year, and the next week is week 1.

    mysql> SELECT WEEK('2008-02-20');
            -> 7
    mysql> SELECT WEEK('2008-02-20',0);
            -> 7
    mysql> SELECT WEEK('2008-02-20',1);
            -> 8
    mysql> SELECT WEEK('2008-12-31',1);
            -> 53
    

    Note that if a date falls in the last week of the previous year, MySQL returns 0 if you do not use 2, 3, 6, or 7 as the optional mode argument:

    mysql> SELECT YEAR('2000-01-01'), WEEK('2000-01-01',0);
            -> 2000, 0
    

    One might argue that WEEK() should return 52 because the given date actually occurs in the 52nd week of 1999. WEEK() returns 0 instead so that the return value is the week number in the given year. This makes use of the WEEK() function reliable when combined with other functions that extract a date part from a date.

    If you prefer a result evaluated with respect to the year that contains the first day of the week for the given date, use 0, 2, 5, or 7 as the optional mode argument.

    mysql> SELECT WEEK('2000-01-01',2);
            -> 52
    

    Alternatively, use the YEARWEEK() function:

    mysql> SELECT YEARWEEK('2000-01-01');
            -> 199952
    mysql> SELECT MID(YEARWEEK('2000-01-01'),5,2);
            -> '52'
    
  • WEEKDAY(date)

    Returns the weekday index for date (0 = Monday, 1 = Tuesday, … 6 = Sunday).

    mysql> SELECT WEEKDAY('2008-02-03 22:23:00');
            -> 6
    mysql> SELECT WEEKDAY('2007-11-06');
            -> 1
    
  • WEEKOFYEAR(date)

    Returns the calendar week of the date as a number in the range from 1 to 53. WEEKOFYEAR() is a compatibility function that is equivalent to WEEK(date,3).

    mysql> SELECT WEEKOFYEAR('2008-02-20');
            -> 8
    
  • YEAR(date)

    Returns the year for date, in the range 1000 to 9999, or 0 for the zero date.

    mysql> SELECT YEAR('1987-01-01');
            -> 1987
    
  • YEARWEEK(date), YEARWEEK(date,mode)

    Returns year and week for a date. The mode argument works exactly like the mode argument to WEEK(). The year in the result may be different from the year in the date argument for the first and the last week of the year.

    mysql> SELECT YEARWEEK('1987-01-01');
            -> 198653
    

    Note that the week number is different from what the WEEK() function would return (0) for optional arguments 0 or 1, as WEEK() then returns the week in the context of the given year.

12.8 What Calendar Is Used By MySQL?

MySQL uses what is known as a proleptic Gregorian calendar.

Every country that has switched from the Julian to the Gregorian calendar has had to discard at least ten days during the switch. To see how this works, consider the month of October 1582, when the first Julian-to-Gregorian switch occurred.

MondayTuesdayWednesdayThursdayFridaySaturdaySunday
1234151617
18192021222324
25262728293031

There are no dates between October 4 and October 15. This discontinuity is called the cutover. Any dates before the cutover are Julian, and any dates following the cutover are Gregorian. Dates during a cutover are nonexistent.

A calendar applied to dates when it was not actually in use is called proleptic. Thus, if we assume there was never a cutover and Gregorian rules always rule, we have a proleptic Gregorian calendar. This is what is used by MySQL, as is required by standard SQL. For this reason, dates prior to the cutover stored as MySQL DATE or DATETIME values must be adjusted to compensate for the difference. It is important to realize that the cutover did not occur at the same time in all countries, and that the later it happened, the more days were lost. For example, in Great Britain, it took place in 1752, when Wednesday September 2 was followed by Thursday September 14. Russia remained on the Julian calendar until 1918, losing 13 days in the process, and what is popularly referred to as its October Revolution occurred in November according to the Gregorian calendar.

For integer expressions, the preceding remarks about expression evaluation apply somewhat differently for expression assignment; for example, in a statement such as this:

CREATE TABLE t SELECT integer_expr;

In this case, the table in the column resulting from the expression has type INT or BIGINT depending on the length of the integer expression. If the maximum length of the expression does not fit in an INT, BIGINT is used instead. The length is taken from the max_length value of the SELECT result set metadata (see Section 23.8.5, “C API Data Structures”). This means that you can force a BIGINT rather than INT by use of a sufficiently long expression:

CREATE TABLE t SELECT 000000000000000000000;

12.9 Full-Text Search Functions

MATCH (col1,col2,...) AGAINST (expr [search_modifier])

search_modifier:
  {
       IN NATURAL LANGUAGE MODE
     | IN NATURAL LANGUAGE MODE WITH QUERY EXPANSION
     | IN BOOLEAN MODE
     | WITH QUERY EXPANSION
  }

MySQL has support for full-text indexing and searching:

  • A full-text index in MySQL is an index of type FULLTEXT.

  • Full-text indexes can be used only with MyISAM tables. (In MySQL 5.6 and up, they can also be used with InnoDB tables.) Full-text indexes can be created only for CHAR, VARCHAR, or TEXT columns.

  • A FULLTEXT index definition can be given in the CREATE TABLE statement when a table is created, or added later using ALTER TABLE or CREATE INDEX.

  • For large data sets, it is much faster to load your data into a table that has no FULLTEXT index and then create the index after that, than to load data into a table that has an existing FULLTEXT index.

Full-text searching is performed using MATCH() ... AGAINST syntax. MATCH() takes a comma-separated list that names the columns to be searched. AGAINST takes a string to search for, and an optional modifier that indicates what type of search to perform. The search string must be a string value that is constant during query evaluation. This rules out, for example, a table column because that can differ for each row.

There are three types of full-text searches:

  • A natural language search interprets the search string as a phrase in natural human language (a phrase in free text). There are no special operators. The stopword list applies. In addition, words that are present in 50% or more of the rows are considered common and do not match.

    Full-text searches are natural language searches if the IN NATURAL LANGUAGE MODE modifier is given or if no modifier is given. For more information, see Section 12.9.1, “Natural Language Full-Text Searches”.

  • A boolean search interprets the search string using the rules of a special query language. The string contains the words to search for. It can also contain operators that specify requirements such that a word must be present or absent in matching rows, or that it should be weighted higher or lower than usual. Common words such as some or then are stopwords and do not match if present in the search string. The IN BOOLEAN MODE modifier specifies a boolean search. For more information, see Section 12.9.2, “Boolean Full-Text Searches”.

  • A query expansion search is a modification of a natural language search. The search string is used to perform a natural language search. Then words from the most relevant rows returned by the search are added to the search string and the search is done again. The query returns the rows from the second search. The IN NATURAL LANGUAGE MODE WITH QUERY EXPANSION or WITH QUERY EXPANSION modifier specifies a query expansion search. For more information, see Section 12.9.3, “Full-Text Searches with Query Expansion”.

Constraints on full-text searching are listed in Section 12.9.5, “Full-Text Restrictions”.

The myisam_ftdump utility can be used to dump the contents of a full-text index. This may be helpful for debugging full-text queries. See Section 4.6.2, “myisam_ftdump — Display Full-Text Index information”.

12.9.1 Natural Language Full-Text Searches

By default or with the IN NATURAL LANGUAGE MODE modifier, the MATCH() function performs a natural language search for a string against a text collection. A collection is a set of one or more columns included in a FULLTEXT index. The search string is given as the argument to AGAINST(). For each row in the table, MATCH() returns a relevance value; that is, a similarity measure between the search string and the text in that row in the columns named in the MATCH() list.

mysql> CREATE TABLE articles (
    ->   id INT UNSIGNED AUTO_INCREMENT NOT NULL PRIMARY KEY,
    ->   title VARCHAR(200),
    ->   body TEXT,
    ->   FULLTEXT (title,body)
    -> ) ENGINE=MyISAM;
Query OK, 0 rows affected (0.00 sec)

mysql> INSERT INTO articles (title,body) VALUES
    -> ('MySQL Tutorial','DBMS stands for DataBase ...'),
    -> ('How To Use MySQL Well','After you went through a ...'),
    -> ('Optimizing MySQL','In this tutorial we will show ...'),
    -> ('1001 MySQL Tricks','1. Never run mysqld as root. 2. ...'),
    -> ('MySQL vs. YourSQL','In the following database comparison ...'),
    -> ('MySQL Security','When configured properly, MySQL ...');
Query OK, 6 rows affected (0.00 sec)
Records: 6  Duplicates: 0  Warnings: 0

mysql> SELECT * FROM articles
    -> WHERE MATCH (title,body)
    -> AGAINST ('database' IN NATURAL LANGUAGE MODE);
+----+-------------------+------------------------------------------+
| id | title             | body                                     |
+----+-------------------+------------------------------------------+
|  5 | MySQL vs. YourSQL | In the following database comparison ... |
|  1 | MySQL Tutorial    | DBMS stands for DataBase ...             |
+----+-------------------+------------------------------------------+
2 rows in set (0.00 sec)

By default, the search is performed in case-insensitive fashion. However, you can perform a case-sensitive full-text search by using a binary collation for the indexed columns. For example, a column that uses the latin1 character set of can be assigned a collation of latin1_bin to make it case sensitive for full-text searches.

When MATCH() is used in a WHERE clause, as in the example shown earlier, the rows returned are automatically sorted with the highest relevance first. Relevance values are nonnegative floating-point numbers. Zero relevance means no similarity. Relevance is computed based on the number of words in the row, the number of unique words in that row, the total number of words in the collection, and the number of documents (rows) that contain a particular word.

To simply count matches, you could use a query like this:

mysql> SELECT COUNT(*) FROM articles
    -> WHERE MATCH (title,body)
    -> AGAINST ('database' IN NATURAL LANGUAGE MODE);
+----------+
| COUNT(*) |
+----------+
|        2 |
+----------+
1 row in set (0.00 sec)

However, you might find it quicker to rewrite the query as follows:

mysql> SELECT
    -> COUNT(IF(MATCH (title,body) AGAINST ('database' IN NATURAL LANGUAGE MODE), 1, NULL))
    -> AS count
    -> FROM articles;
+-------+
| count |
+-------+
|     2 |
+-------+
1 row in set (0.03 sec)

The first query sorts the results by relevance whereas the second does not. However, the second query performs a full table scan and the first does not. The first may be faster if the search matches few rows; otherwise, the second may be faster because it would read many rows anyway.

For natural-language full-text searches, it is a requirement that the columns named in the MATCH() function be the same columns included in some FULLTEXT index in your table. For the preceding query, note that the columns named in the MATCH() function (title and body) are the same as those named in the definition of the article table's FULLTEXT index. If you wanted to search the title or body separately, you would need to create separate FULLTEXT indexes for each column.

It is also possible to perform a boolean search or a search with query expansion. These search types are described in Section 12.9.2, “Boolean Full-Text Searches”, and Section 12.9.3, “Full-Text Searches with Query Expansion”.

A full-text search that uses an index can name columns only from a single table in the MATCH() clause because an index cannot span multiple tables. A boolean search can be done in the absence of an index (albeit more slowly), in which case it is possible to name columns from multiple tables.

The preceding example is a basic illustration that shows how to use the MATCH() function where rows are returned in order of decreasing relevance. The next example shows how to retrieve the relevance values explicitly. Returned rows are not ordered because the SELECT statement includes neither WHERE nor ORDER BY clauses:

mysql> SELECT id, MATCH (title,body)
    -> AGAINST ('Tutorial' IN NATURAL LANGUAGE MODE) AS score
    -> FROM articles;
+----+------------------+
| id | score            |
+----+------------------+
|  1 | 0.65545833110809 |
|  2 |                0 |
|  3 | 0.66266459226608 |
|  4 |                0 |
|  5 |                0 |
|  6 |                0 |
+----+------------------+
6 rows in set (0.00 sec)

The following example is more complex. The query returns the relevance values and it also sorts the rows in order of decreasing relevance. To achieve this result, specify MATCH() twice: once in the SELECT list and once in the WHERE clause. This causes no additional overhead, because the MySQL optimizer notices that the two MATCH() calls are identical and invokes the full-text search code only once.

mysql> SELECT id, body, MATCH (title,body) AGAINST
    -> ('Security implications of running MySQL as root'
    -> IN NATURAL LANGUAGE MODE) AS score
    -> FROM articles WHERE MATCH (title,body) AGAINST
    -> ('Security implications of running MySQL as root'
    -> IN NATURAL LANGUAGE MODE);
+----+-------------------------------------+-----------------+
| id | body                                | score           |
+----+-------------------------------------+-----------------+
|  4 | 1. Never run mysqld as root. 2. ... | 1.5219271183014 |
|  6 | When configured properly, MySQL ... | 1.3114095926285 |
+----+-------------------------------------+-----------------+
2 rows in set (0.00 sec)

The MySQL FULLTEXT implementation regards any sequence of true word characters (letters, digits, and underscores) as a word. That sequence may also contain apostrophes ('), but not more than one in a row. This means that aaa'bbb is regarded as one word, but aaa''bbb is regarded as two words. Apostrophes at the beginning or the end of a word are stripped by the FULLTEXT parser; 'aaa'bbb' would be parsed as aaa'bbb.

The FULLTEXT parser determines where words start and end by looking for certain delimiter characters; for example,   (space), , (comma), and . (period). If words are not separated by delimiters (as in, for example, Chinese), the FULLTEXT parser cannot determine where a word begins or ends. To be able to add words or other indexed terms in such languages to a FULLTEXT index, you must preprocess them so that they are separated by some arbitrary delimiter such as ".

In MySQL 5.5, it is possible to write a plugin that replaces the built-in full-text parser. For details, see Section 24.2, “The MySQL Plugin API”. For example parser plugin source code, see the plugin/fulltext directory of a MySQL source distribution.

Some words are ignored in full-text searches:

  • Any word that is too short is ignored. The default minimum length of words that are found by full-text searches is four characters.

  • Words in the stopword list are ignored. A stopword is a word such as the or some that is so common that it is considered to have zero semantic value. There is a built-in stopword list, but it can be overwritten by a user-defined list.

The default stopword list is given in Section 12.9.4, “Full-Text Stopwords”. The default minimum word length and stopword list can be changed as described in Section 12.9.6, “Fine-Tuning MySQL Full-Text Search”.

Every correct word in the collection and in the query is weighted according to its significance in the collection or query. Consequently, a word that is present in many documents has a lower weight (and may even have a zero weight), because it has lower semantic value in this particular collection. Conversely, if the word is rare, it receives a higher weight. The weights of the words are combined to compute the relevance of the row.

Such a technique works best with large collections (in fact, it was carefully tuned this way). For very small tables, word distribution does not adequately reflect their semantic value, and this model may sometimes produce bizarre results. For example, although the word MySQL is present in every row of the articles table shown earlier, a search for the word produces no results:

mysql> SELECT * FROM articles
    -> WHERE MATCH (title,body)
    -> AGAINST ('MySQL' IN NATURAL LANGUAGE MODE);
Empty set (0.00 sec)

The search result is empty because the word MySQL is present in at least 50% of the rows. As such, it is effectively treated as a stopword. For large data sets, this is the most desirable behavior: A natural language query should not return every second row from a 1GB table. For small data sets, it may be less desirable.

A word that matches half of the rows in a table is less likely to locate relevant documents. In fact, it most likely finds plenty of irrelevant documents. We all know this happens far too often when we are trying to find something on the Internet with a search engine. It is with this reasoning that rows containing the word are assigned a low semantic value for the particular data set in which they occur. A given word may reach the 50% threshold in one data set but not another.

The 50% threshold has a significant implication when you first try full-text searching to see how it works: If you create a table and insert only one or two rows of text into it, every word in the text occurs in at least 50% of the rows. As a result, no search returns any results. Be sure to insert at least three rows, and preferably many more. Users who need to bypass the 50% limitation can use the boolean search mode; see Section 12.9.2, “Boolean Full-Text Searches”.

12.9.2 Boolean Full-Text Searches

MySQL can perform boolean full-text searches using the IN BOOLEAN MODE modifier. With this modifier, certain characters have special meaning at the beginning or end of words in the search string. In the following query, the + and - operators indicate that a word is required to be present or absent, respectively, for a match to occur. Thus, the query retrieves all the rows that contain the word MySQL but that do not contain the word YourSQL:

mysql> SELECT * FROM articles WHERE MATCH (title,body)
    -> AGAINST ('+MySQL -YourSQL' IN BOOLEAN MODE);
+----+-----------------------+-------------------------------------+
| id | title                 | body                                |
+----+-----------------------+-------------------------------------+
|  1 | MySQL Tutorial        | DBMS stands for DataBase ...        |
|  2 | How To Use MySQL Well | After you went through a ...        |
|  3 | Optimizing MySQL      | In this tutorial we will show ...   |
|  4 | 1001 MySQL Tricks     | 1. Never run mysqld as root. 2. ... |
|  6 | MySQL Security        | When configured properly, MySQL ... |
+----+-----------------------+-------------------------------------+
Note

In implementing this feature, MySQL uses what is sometimes referred to as implied Boolean logic, in which

  • + stands for AND

  • - stands for NOT

  • [no operator] implies OR

Boolean full-text searches have these characteristics:

  • They do not use the 50% threshold.

  • They do not automatically sort rows in order of decreasing relevance. You can see this from the preceding query result: The row with the highest relevance is the one that contains MySQL twice, but it is listed last, not first.

  • They can work even without a FULLTEXT index, although a search executed in this fashion would be quite slow.

  • The minimum and maximum word length full-text parameters apply.

  • The stopword list applies.

The boolean full-text search capability supports the following operators:

  • +

    A leading plus sign indicates that this word must be present in each row that is returned.

  • -

    A leading minus sign indicates that this word must not be present in any of the rows that are returned.

    Note: The - operator acts only to exclude rows that are otherwise matched by other search terms. Thus, a boolean-mode search that contains only terms preceded by - returns an empty result. It does not return all rows except those containing any of the excluded terms.

  • (no operator)

    By default (when neither + nor - is specified) the word is optional, but the rows that contain it are rated higher. This mimics the behavior of MATCH() ... AGAINST() without the IN BOOLEAN MODE modifier.

  • > <

    These two operators are used to change a word's contribution to the relevance value that is assigned to a row. The > operator increases the contribution and the < operator decreases it. See the example following this list.

  • ( )

    Parentheses group words into subexpressions. Parenthesized groups can be nested.

  • ~

    A leading tilde acts as a negation operator, causing the word's contribution to the row's relevance to be negative. This is useful for marking noise words. A row containing such a word is rated lower than others, but is not excluded altogether, as it would be with the - operator.

  • *

    The asterisk serves as the truncation (or wildcard) operator. Unlike the other operators, it should be appended to the word to be affected. Words match if they begin with the word preceding the * operator.

    If a word is specified with the truncation operator, it is not stripped from a boolean query, even if it is too short (as determined from the ft_min_word_len setting) or a stopword. This occurs because the word is not seen as too short or a stopword, but as a prefix that must be present in the document in the form of a word that begins with the prefix. Suppose that ft_min_word_len=4. Then a search for '+word +the*' will likely return fewer rows than a search for '+word +the':

    • The former query remains as is and requires both word and the* (a word starting with the) to be present in the document.

    • The latter query is transformed to +word (requiring only word to be present). the is both too short and a stopword, and either condition is enough to cause it to be ignored.

  • "

    A phrase that is enclosed within double quote (") characters matches only rows that contain the phrase literally, as it was typed. The full-text engine splits the phrase into words and performs a search in the FULLTEXT index for the words. Nonword characters need not be matched exactly: Phrase searching requires only that matches contain exactly the same words as the phrase and in the same order. For example, "test phrase" matches "test, phrase".

    If the phrase contains no words that are in the index, the result is empty. For example, if all words are either stopwords or shorter than the minimum length of indexed words, the result is empty.

The following examples demonstrate some search strings that use boolean full-text operators:

  • 'apple banana'

    Find rows that contain at least one of the two words.

  • '+apple +juice'

    Find rows that contain both words.

  • '+apple macintosh'

    Find rows that contain the word apple, but rank rows higher if they also contain macintosh.

  • '+apple -macintosh'

    Find rows that contain the word apple but not macintosh.

  • '+apple ~macintosh'

    Find rows that contain the word apple, but if the row also contains the word macintosh, rate it lower than if row does not. This is softer than a search for '+apple -macintosh', for which the presence of macintosh causes the row not to be returned at all.

  • '+apple +(>turnover <strudel)'

    Find rows that contain the words apple and turnover, or apple and strudel (in any order), but rank apple turnover higher than apple strudel.

  • 'apple*'

    Find rows that contain words such as apple, apples, applesauce, or applet.

  • '"some words"'

    Find rows that contain the exact phrase some words (for example, rows that contain some words of wisdom but not some noise words). Note that the " characters that enclose the phrase are operator characters that delimit the phrase. They are not the quotation marks that enclose the search string itself.

12.9.3 Full-Text Searches with Query Expansion

Full-text search supports query expansion (and in particular, its variant blind query expansion). This is generally useful when a search phrase is too short, which often means that the user is relying on implied knowledge that the full-text search engine lacks. For example, a user searching for database may really mean that MySQL, Oracle, DB2, and RDBMS all are phrases that should match databases and should be returned, too. This is implied knowledge.

Blind query expansion (also known as automatic relevance feedback) is enabled by adding WITH QUERY EXPANSION or IN NATURAL LANGUAGE MODE WITH QUERY EXPANSION following the search phrase. It works by performing the search twice, where the search phrase for the second search is the original search phrase concatenated with the few most highly relevant documents from the first search. Thus, if one of these documents contains the word databases and the word MySQL, the second search finds the documents that contain the word MySQL even if they do not contain the word database. The following example shows this difference:

mysql> SELECT * FROM articles
    -> WHERE MATCH (title,body)
    -> AGAINST ('database' IN NATURAL LANGUAGE MODE);
+----+-------------------+------------------------------------------+
| id | title             | body                                     |
+----+-------------------+------------------------------------------+
|  5 | MySQL vs. YourSQL | In the following database comparison ... |
|  1 | MySQL Tutorial    | DBMS stands for DataBase ...             |
+----+-------------------+------------------------------------------+
2 rows in set (0.00 sec)

mysql> SELECT * FROM articles
    -> WHERE MATCH (title,body)
    -> AGAINST ('database' WITH QUERY EXPANSION);
+----+-------------------+------------------------------------------+
| id | title             | body                                     |
+----+-------------------+------------------------------------------+
|  1 | MySQL Tutorial    | DBMS stands for DataBase ...             |
|  5 | MySQL vs. YourSQL | In the following database comparison ... |
|  3 | Optimizing MySQL  | In this tutorial we will show ...        |
+----+-------------------+------------------------------------------+
3 rows in set (0.00 sec)

Another example could be searching for books by Georges Simenon about Maigret, when a user is not sure how to spell Maigret. A search for Megre and the reluctant witnesses finds only Maigret and the Reluctant Witnesses without query expansion. A search with query expansion finds all books with the word Maigret on the second pass.

Note

Because blind query expansion tends to increase noise significantly by returning nonrelevant documents, it is meaningful to use only when a search phrase is rather short.

12.9.4 Full-Text Stopwords

The stopword list is loaded and searched for full-text queries using the server character set and collation (the values of the character_set_server and collation_server system variables). False hits or misses may occur for stopword lookups if the stopword file or columns used for full-text indexing or searches have a character set or collation different from character_set_server or collation_server.

Case sensitivity of stopword lookups depends on the server collation. For example, lookups are case insensitive if the collation is latin1_swedish_ci, whereas lookups are case sensitive if the collation is latin1_general_cs or latin1_bin.

As of MySQL 5.5.6, the stopword file is loaded and searched using latin1 if character_set_server is ucs2, utf16, or utf32. If any table was created with FULLTEXT indexes while the server character set was ucs2, utf16, or utf32, it should be repaired using this statement:

REPAIR TABLE tbl_name QUICK;

The following table shows the default list of full-text stopwords. In a MySQL source distribution, you can find this list in the storage/myisam/ft_static.c file.

a'sableaboutaboveaccording
accordinglyacrossactuallyafterafterwards
againagainstain'tallallow
allowsalmostalonealongalready
alsoalthoughalwaysamamong
amongstanandanotherany
anybodyanyhowanyoneanythinganyway
anywaysanywhereapartappearappreciate
appropriatearearen'taroundas
asideaskaskingassociatedat
availableawayawfullybebecame
becausebecomebecomesbecomingbeen
beforebeforehandbehindbeingbelieve
belowbesidebesidesbestbetter
betweenbeyondbothbriefbut
byc'monc'scamecan
can'tcannotcantcausecauses
certaincertainlychangesclearlyco
comcomecomesconcerningconsequently
considerconsideringcontaincontainingcontains
correspondingcouldcouldn'tcoursecurrently
definitelydescribeddespitediddidn't
differentdodoesdoesn'tdoing
don'tdonedowndownwardsduring
eacheduegeighteither
elseelsewhereenoughentirelyespecially
etetceveneverevery
everybodyeveryoneeverythingeverywhereex
exactlyexampleexceptfarfew
fifthfirstfivefollowedfollowing
followsforformerformerlyforth
fourfromfurtherfurthermoreget
getsgettinggivengivesgo
goesgoinggonegotgotten
greetingshadhadn'thappenshardly
hashasn'thavehaven'thaving
hehe'shellohelphence
herherehere'shereafterhereby
hereinhereuponhersherselfhi
himhimselfhishitherhopefully
howhowbeithoweveri'di'll
i'mi'veieifignored
immediateininasmuchincindeed
indicateindicatedindicatesinnerinsofar
insteadintoinwardisisn't
itit'dit'llit'sits
itselfjustkeepkeepskept
knowknownknowslastlately
laterlatterlatterlyleastless
lestletlet'slikeliked
likelylittlelooklookinglooks
ltdmainlymanymaymaybe
memeanmeanwhilemerelymight
moremoreovermostmostlymuch
mustmymyselfnamenamely
ndnearnearlynecessaryneed
needsneitherneverneverthelessnew
nextninenonobodynon
nonenoonenornormallynot
nothingnovelnownowhereobviously
ofoffoftenohok
okayoldononceone
onesonlyontoorother
othersotherwiseoughtourours
ourselvesoutoutsideoveroverall
ownparticularparticularlyperperhaps
placedpleasepluspossiblepresumably
probablyprovidesquequiteqv
ratherrdrereallyreasonably
regardingregardlessregardsrelativelyrespectively
rightsaidsamesawsay
sayingsayssecondsecondlysee
seeingseemseemedseemingseems
seenselfselvessensiblesent
seriousseriouslysevenseveralshall
sheshouldshouldn'tsincesix
sosomesomebodysomehowsomeone
somethingsometimesometimessomewhatsomewhere
soonsorryspecifiedspecifyspecifying
stillsubsuchsupsure
t'staketakentelltends
ththanthankthanksthanx
thatthat'sthatsthetheir
theirsthemthemselvesthenthence
therethere'sthereaftertherebytherefore
thereintheresthereuponthesethey
they'dthey'llthey'rethey'vethink
thirdthisthoroughthoroughlythose
thoughthreethroughthroughoutthru
thustotogethertootook
towardtowardstriedtriestruly
trytryingtwicetwoun
underunfortunatelyunlessunlikelyuntil
untoupuponususe
usedusefulusesusingusually
valuevariousveryviaviz
vswantwantswaswasn't
waywewe'dwe'llwe're
we'vewelcomewellwentwere
weren'twhatwhat'swhateverwhen
whencewheneverwherewhere'swhereafter
whereaswherebywhereinwhereuponwherever
whetherwhichwhilewhitherwho
who'swhoeverwholewhomwhose
whywillwillingwishwith
withinwithoutwon'twonderwould
wouldn'tyesyetyouyou'd
you'llyou'reyou'veyouryours
yourselfyourselveszero  

12.9.5 Full-Text Restrictions

  • Full-text searches are supported for MyISAM tables only. (In MySQL 5.6 and up, they can also be used with InnoDB tables.)

  • Full-text searches are not supported for partitioned tables. See Section 19.5, “Restrictions and Limitations on Partitioning”.

  • Full-text searches can be used with most multibyte character sets. The exception is that for Unicode, the utf8 character set can be used, but not the ucs2 character set. However, although FULLTEXT indexes on ucs2 columns cannot be used, you can perform IN BOOLEAN MODE searches on a ucs2 column that has no such index.

    The remarks for utf8 also apply to utf8mb4, and the remarks for ucs2 also apply to utf16 and utf32.

  • Ideographic languages such as Chinese and Japanese do not have word delimiters. Therefore, the FULLTEXT parser cannot determine where words begin and end in these and other such languages. The implications of this and some workarounds for the problem are described in Section 12.9, “Full-Text Search Functions”.

  • Although the use of multiple character sets within a single table is supported, all columns in a FULLTEXT index must use the same character set and collation.

  • The MATCH() column list must match exactly the column list in some FULLTEXT index definition for the table, unless this MATCH() is IN BOOLEAN MODE. Boolean-mode searches can be done on nonindexed columns, although they are likely to be slow.

  • The argument to AGAINST() must be a string value that is constant during query evaluation. This rules out, for example, a table column because that can differ for each row.

  • Index hints are more limited for FULLTEXT searches than for non-FULLTEXT searches. See Section 13.2.9.3, “Index Hint Syntax”.

12.9.6 Fine-Tuning MySQL Full-Text Search

MySQL's full-text search capability has few user-tunable parameters. You can exert more control over full-text searching behavior if you have a MySQL source distribution because some changes require source code modifications. See Section 2.9, “Installing MySQL from Source”.

Full-text search is carefully tuned for the most effectiveness. Modifying the default behavior in most cases can actually decrease effectiveness. Do not alter the MySQL sources unless you know what you are doing.

Most full-text variables described in this section must be set at server startup time. A server restart is required to change them; they cannot be modified while the server is running.

Some variable changes require that you rebuild the FULLTEXT indexes in your tables. Instructions for doing so are given later in this section.

  • The minimum and maximum lengths of words to be indexed are defined by the ft_min_word_len and ft_max_word_len system variables. (See Section 5.1.4, “Server System Variables”.) The default minimum value is four characters; the default maximum is version dependent. If you change either value, you must rebuild your FULLTEXT indexes. For example, if you want three-character words to be searchable, you can set the ft_min_word_len variable by putting the following lines in an option file:

    [mysqld]
    ft_min_word_len=3
    

    Then restart the server and rebuild your FULLTEXT indexes. Note particularly the remarks regarding myisamchk in the instructions following this list.

  • To override the default stopword list, set the ft_stopword_file system variable. (See Section 5.1.4, “Server System Variables”.) The variable value should be the path name of the file containing the stopword list, or the empty string to disable stopword filtering. The server looks for the file in the data directory unless an absolute path name is given to specify a different directory. After changing the value of this variable or the contents of the stopword file, restart the server and rebuild your FULLTEXT indexes.

    The stopword list is free-form. That is, you may use any nonalphanumeric character such as newline, space, or comma to separate stopwords. Exceptions are the underscore character (_) and a single apostrophe (') which are treated as part of a word. The character set of the stopword list is the server's default character set; see Section 10.1.3.1, “Server Character Set and Collation”.

  • The 50% threshold for natural language searches is determined by the particular weighting scheme chosen. To disable it, look for the following line in storage/myisam/ftdefs.h:

    #define GWS_IN_USE GWS_PROB
    

    Change that line to this:

    #define GWS_IN_USE GWS_FREQ
    

    Then recompile MySQL. There is no need to rebuild the indexes in this case.

    Note

    By making this change, you severely decrease MySQL's ability to provide adequate relevance values for the MATCH() function. If you really need to search for such common words, it would be better to search using IN BOOLEAN MODE instead, which does not observe the 50% threshold.

  • To change the operators used for boolean full-text searches, set the ft_boolean_syntax system variable. This variable can be changed while the server is running, but you must have the SUPER privilege to do so. No rebuilding of indexes is necessary in this case. See Section 5.1.4, “Server System Variables”, which describes the rules governing how to set this variable.

  • If you want to change the set of characters that are considered word characters, you can do so in several ways, as described in the following list. After making the modification, you must rebuild the indexes for each table that contains any FULLTEXT indexes. Suppose that you want to treat the hyphen character ('-') as a word character. Use one of these methods:

    • Modify the MySQL source: In storage/myisam/ftdefs.h, see the true_word_char() and misc_word_char() macros. Add '-' to one of those macros and recompile MySQL.

    • Modify a character set file: This requires no recompilation. The true_word_char() macro uses a character type table to distinguish letters and numbers from other characters. . You can edit the contents of the <ctype><map> array in one of the character set XML files to specify that '-' is a letter. Then use the given character set for your FULLTEXT indexes. For information about the <ctype><map> array format, see Section 10.3.1, “Character Definition Arrays”.

    • Add a new collation for the character set used by the indexed columns, and alter the columns to use that collation. For general information about adding collations, see Section 10.4, “Adding a Collation to a Character Set”. For an example specific to full-text indexing, see Section 12.9.7, “Adding a Collation for Full-Text Indexing”.

If you modify full-text variables that affect indexing (ft_min_word_len, ft_max_word_len, or ft_stopword_file), or if you change the stopword file itself, you must rebuild your FULLTEXT indexes after making the changes and restarting the server. To rebuild the indexes in this case, it is sufficient to do a QUICK repair operation:

mysql> REPAIR TABLE tbl_name QUICK;

Alternatively, use ALTER TABLE with the DROP INDEX and ADD INDEX options to drop and re-create each FULLTEXT index. In some cases, this may be faster than a repair operation.

Each table that contains any FULLTEXT index must be repaired as just shown. Otherwise, queries for the table may yield incorrect results, and modifications to the table will cause the server to see the table as corrupt and in need of repair.

If you use myisamchk to perform an operation that modifies table indexes (such as repair or analyze), the FULLTEXT indexes are rebuilt using the default full-text parameter values for minimum word length, maximum word length, and stopword file unless you specify otherwise. This can result in queries failing.

The problem occurs because these parameters are known only by the server. They are not stored in MyISAM index files. To avoid the problem if you have modified the minimum or maximum word length or stopword file values used by the server, specify the same ft_min_word_len, ft_max_word_len, and ft_stopword_file values for myisamchk that you use for mysqld. For example, if you have set the minimum word length to 3, you can repair a table with myisamchk like this:

shell> myisamchk --recover --ft_min_word_len=3 tbl_name.MYI

To ensure that myisamchk and the server use the same values for full-text parameters, place each one in both the [mysqld] and [myisamchk] sections of an option file:

[mysqld]
ft_min_word_len=3

[myisamchk]
ft_min_word_len=3

An alternative to using myisamchk for index modification is to use the REPAIR TABLE, ANALYZE TABLE, OPTIMIZE TABLE, or ALTER TABLE statements. These statements are performed by the server, which knows the proper full-text parameter values to use.

12.9.7 Adding a Collation for Full-Text Indexing

This section describes how to add a new collation for full-text searches. The sample collation is like latin1_swedish_ci but treats the '-' character as a letter rather than as a punctuation character so that it can be indexed as a word character. General information about adding collations is given in Section 10.4, “Adding a Collation to a Character Set”; it is assumed that you have read it and are familiar with the files involved.

To add a collation for full-text indexing, use this procedure:

  1. Add a collation to the Index.xml file. The collation ID must be unused, so choose a value different from 62 if that ID is already taken on your system.

    <charset name="latin1">
    ...
    <collation name="latin1_fulltext_ci" id="62"/>
    </charset>
    
  2. Declare the sort order for the collation in the latin1.xml file. In this case, the order can be copied from latin1_swedish_ci:

    <collation name="latin1_fulltext_ci">
    <map>
    00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F
    10 11 12 13 14 15 16 17 18 19 1A 1B 1C 1D 1E 1F
    20 21 22 23 24 25 26 27 28 29 2A 2B 2C 2D 2E 2F
    30 31 32 33 34 35 36 37 38 39 3A 3B 3C 3D 3E 3F
    40 41 42 43 44 45 46 47 48 49 4A 4B 4C 4D 4E 4F
    50 51 52 53 54 55 56 57 58 59 5A 5B 5C 5D 5E 5F
    60 41 42 43 44 45 46 47 48 49 4A 4B 4C 4D 4E 4F
    50 51 52 53 54 55 56 57 58 59 5A 7B 7C 7D 7E 7F
    80 81 82 83 84 85 86 87 88 89 8A 8B 8C 8D 8E 8F
    90 91 92 93 94 95 96 97 98 99 9A 9B 9C 9D 9E 9F
    A0 A1 A2 A3 A4 A5 A6 A7 A8 A9 AA AB AC AD AE AF
    B0 B1 B2 B3 B4 B5 B6 B7 B8 B9 BA BB BC BD BE BF
    41 41 41 41 5C 5B 5C 43 45 45 45 45 49 49 49 49
    44 4E 4F 4F 4F 4F 5D D7 D8 55 55 55 59 59 DE DF
    41 41 41 41 5C 5B 5C 43 45 45 45 45 49 49 49 49
    44 4E 4F 4F 4F 4F 5D F7 D8 55 55 55 59 59 DE FF
    </map>
    </collation>
    
  3. Modify the ctype array in latin1.xml. Change the value corresponding to 0x2D (which is the code for the '-' character) from 10 (punctuation) to 01 (small letter). In the following array, this is the element in the fourth row down, third value from the end.

    <ctype>
    <map>
    00
    20 20 20 20 20 20 20 20 20 28 28 28 28 28 20 20
    20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20
    48 10 10 10 10 10 10 10 10 10 10 10 10 01 10 10
    84 84 84 84 84 84 84 84 84 84 10 10 10 10 10 10
    10 81 81 81 81 81 81 01 01 01 01 01 01 01 01 01
    01 01 01 01 01 01 01 01 01 01 01 10 10 10 10 10
    10 82 82 82 82 82 82 02 02 02 02 02 02 02 02 02
    02 02 02 02 02 02 02 02 02 02 02 10 10 10 10 20
    10 00 10 02 10 10 10 10 10 10 01 10 01 00 01 00
    00 10 10 10 10 10 10 10 10 10 02 10 02 00 02 01
    48 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10
    10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10
    01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01
    01 01 01 01 01 01 01 10 01 01 01 01 01 01 01 02
    02 02 02 02 02 02 02 02 02 02 02 02 02 02 02 02
    02 02 02 02 02 02 02 10 02 02 02 02 02 02 02 02
    </map>
    </ctype>
    
  4. Restart the server.

  5. To employ the new collation, include it in the definition of columns that are to use it:

    mysql> DROP TABLE IF EXISTS t1;
    Query OK, 0 rows affected (0.13 sec)
    
    mysql> CREATE TABLE t1 (
        -> a TEXT CHARACTER SET latin1 COLLATE latin1_fulltext_ci,
        -> FULLTEXT INDEX(a)
        -> ) ENGINE=MyISAM;
    Query OK, 0 rows affected (0.47 sec)
    
  6. Test the collation to verify that hyphen is considered as a word character:

    mysql> INSERT INTO t1 VALUEs ('----'),('....'),('abcd');
    Query OK, 3 rows affected (0.22 sec)
    Records: 3  Duplicates: 0  Warnings: 0
    
    mysql> SELECT * FROM t1 WHERE MATCH a AGAINST ('----' IN BOOLEAN MODE);
    +------+
    | a    |
    +------+
    | ---- |
    +------+
    1 row in set (0.00 sec)
    

12.10 Cast Functions and Operators

Table 12.14 Cast Functions

NameDescription
BINARYCast a string to a binary string
CAST()Cast a value as a certain type
CONVERT()Cast a value as a certain type

  • BINARY

    The BINARY operator casts the string following it to a binary string. This is an easy way to force a column comparison to be done byte by byte rather than character by character. This causes the comparison to be case sensitive even if the column is not defined as BINARY or BLOB. BINARY also causes trailing spaces to be significant.

    mysql> SELECT 'a' = 'A';
            -> 1
    mysql> SELECT BINARY 'a' = 'A';
            -> 0
    mysql> SELECT 'a' = 'a ';
            -> 1
    mysql> SELECT BINARY 'a' = 'a ';
            -> 0
    

    In a comparison, BINARY affects the entire operation; it can be given before either operand with the same result.

    BINARY str is shorthand for CAST(str AS BINARY).

    Note that in some contexts, if you cast an indexed column to BINARY, MySQL is not able to use the index efficiently.

  • CAST(expr AS type)

    The CAST() function takes an expression of any type and produces a result value of a specified type, similar to CONVERT(). See the description of CONVERT() for more information.

  • CONVERT(expr,type), CONVERT(expr USING transcoding_name)

    The CONVERT() and CAST() functions take an expression of any type and produce a result value of a specified type.

    The type for the result can be one of the following values:

    • BINARY[(N)]

    • CHAR[(N)]

    • DATE

    • DATETIME

    • DECIMAL[(M[,D])]

    • SIGNED [INTEGER]

    • TIME

    • UNSIGNED [INTEGER]

    BINARY produces a string with the BINARY data type. See Section 11.4.2, “The BINARY and VARBINARY Types” for a description of how this affects comparisons. If the optional length N is given, BINARY(N) causes the cast to use no more than N bytes of the argument. Values shorter than N bytes are padded with 0x00 bytes to a length of N.

    CHAR(N) causes the cast to use no more than N characters of the argument.

    CAST() and CONVERT(... USING ...) are standard SQL syntax. The non-USING form of CONVERT() is ODBC syntax.

    CONVERT() with USING is used to convert data between different character sets. In MySQL, transcoding names are the same as the corresponding character set names. For example, this statement converts the string 'abc' in the default character set to the corresponding string in the utf8 character set:

    SELECT CONVERT('abc' USING utf8);
    

Normally, you cannot compare a BLOB value or other binary string in case-insensitive fashion because binary strings have no character set, and thus no concept of lettercase. To perform a case-insensitive comparison, use the CONVERT() function to convert the value to a nonbinary string. Comparisons of the result use the string collation. For example, if the character set of the result has a case-insensitive collation, a LIKE operation is not case sensitive:

SELECT 'A' LIKE CONVERT(blob_col USING latin1) FROM tbl_name;

To use a different character set, substitute its name for latin1 in the preceding statement. To specify a particular collation for the converted string, use a COLLATE clause following the CONVERT() call, as described in Section 10.1.9.2, “CONVERT() and CAST()”. For example, to use latin1_german1_ci:

SELECT 'A' LIKE CONVERT(blob_col USING latin1) COLLATE latin1_german1_ci
  FROM tbl_name;

CONVERT() can be used more generally for comparing strings that are represented in different character sets.

LOWER() (and UPPER()) are ineffective when applied to binary strings (BINARY, VARBINARY, BLOB). To perform lettercase conversion, convert the string to a nonbinary string:

mysql> SET @str = BINARY 'New York';
mysql> SELECT LOWER(@str), LOWER(CONVERT(@str USING latin1));
+-------------+-----------------------------------+
| LOWER(@str) | LOWER(CONVERT(@str USING latin1)) |
+-------------+-----------------------------------+
| New York    | new york                          |
+-------------+-----------------------------------+

The cast functions are useful when you want to create a column with a specific type in a CREATE TABLE ... SELECT statement:

CREATE TABLE new_table SELECT CAST('2000-01-01' AS DATE);

The functions also can be useful for sorting ENUM columns in lexical order. Normally, sorting of ENUM columns occurs using the internal numeric values. Casting the values to CHAR results in a lexical sort:

SELECT enum_col FROM tbl_name ORDER BY CAST(enum_col AS CHAR);

CAST(str AS BINARY) is the same thing as BINARY str. CAST(expr AS CHAR) treats the expression as a string with the default character set.

CAST() also changes the result if you use it as part of a more complex expression such as CONCAT('Date: ',CAST(NOW() AS DATE)).

You should not use CAST() to extract data in different formats but instead use string functions like LEFT() or EXTRACT(). See Section 12.7, “Date and Time Functions”.

To cast a string to a numeric value in numeric context, you normally do not have to do anything other than to use the string value as though it were a number:

mysql> SELECT 1+'1';
       -> 2

If you use a string in an arithmetic operation, it is converted to a floating-point number during expression evaluation.

If you use a number in string context, the number automatically is converted to a string:

mysql> SELECT CONCAT('hello you ',2);
        -> 'hello you 2'

For information about implicit conversion of numbers to strings, see Section 12.2, “Type Conversion in Expression Evaluation”.

When using an explicit CAST() on a TIMESTAMP value in a statement that does not select from any tables, the value is treated by MySQL 5.5 as a string prior to performing any conversion. This results in the value being truncated when casting to a numeric type, as shown here:

mysql> SELECT CAST(TIMESTAMP '2014-09-08 18:07:54' AS SIGNED);
+-------------------------------------------------+
| CAST(TIMESTAMP '2014-09-08 18:07:54' AS SIGNED) |
+-------------------------------------------------+
|                                            2014 |
+-------------------------------------------------+
1 row in set, 1 warning (0.00 sec)

mysql> SHOW WARNINGS;
+---------+------+----------------------------------------------------------+
| Level   | Code | Message                                                  |
+---------+------+----------------------------------------------------------+
| Warning | 1292 | Truncated incorrect INTEGER value: '2014-09-08 18:07:54' |
+---------+------+----------------------------------------------------------+
1 row in set (0.00 sec)

This does not apply when selecting rows from a table, as shown here:

mysql> USE test;

Database changed
mysql> CREATE TABLE c_test (col TIMESTAMP);
Query OK, 0 rows affected (0.07 sec)

mysql> INSERT INTO c_test VALUES ('2014-09-08 18:07:54');
Query OK, 1 row affected (0.05 sec)

mysql> SELECT col, CAST(col AS UNSIGNED) AS c_col FROM c_test;
+---------------------+----------------+
| col                 | c_col          |
+---------------------+----------------+
| 2014-09-08 18:07:54 | 20140908180754 |
+---------------------+----------------+
1 row in set (0.00 sec)

This is a known issue which is resolved in MySQL 5.6.

MySQL supports arithmetic with both signed and unsigned 64-bit values. If you are using numeric operators (such as + or -) and one of the operands is an unsigned integer, the result is unsigned by default (see Section 12.6.1, “Arithmetic Operators”). You can override this by using the SIGNED or UNSIGNED cast operator to cast a value to a signed or unsigned 64-bit integer, respectively.

mysql> SELECT CAST(1-2 AS UNSIGNED)
        -> 18446744073709551615
mysql> SELECT CAST(CAST(1-2 AS UNSIGNED) AS SIGNED);
        -> -1

If either operand is a floating-point value, the result is a floating-point value and is not affected by the preceding rule. (In this context, DECIMAL column values are regarded as floating-point values.)

mysql> SELECT CAST(1 AS UNSIGNED) - 2.0;
        -> -1.0

The SQL mode affects the result of conversion operations. Examples:

  • If you convert a zero date string to a date, CONVERT() and CAST() return NULL and produce a warning when the NO_ZERO_DATE SQL mode is enabled.

  • For integer subtraction, if the NO_UNSIGNED_SUBTRACTION SQL mode is enabled, the subtraction result is signed even if any operand is unsigned.

For more information, see Section 5.1.7, “Server SQL Modes”.

12.11 XML Functions

Table 12.15 XML Functions

NameDescription
ExtractValue()Extracts a value from an XML string using XPath notation
UpdateXML()Return replaced XML fragment

This section discusses XML and related functionality in MySQL.

Note

It is possible to obtain XML-formatted output from MySQL in the mysql and mysqldump clients by invoking them with the --xml option. See Section 4.5.1, “mysql — The MySQL Command-Line Tool”, and Section 4.5.4, “mysqldump — A Database Backup Program”.

Two functions providing basic XPath 1.0 (XML Path Language, version 1.0) capabilities are available. Some basic information about XPath syntax and usage is provided later in this section; however, an in-depth discussion of these topics is beyond the scope of this Manual, and you should refer to the XML Path Language (XPath) 1.0 standard for definitive information. A useful resource for those new to XPath or who desire a refresher in the basics is the Zvon.org XPath Tutorial, which is available in several languages.

Note

These functions remain under development. We continue to improve these and other aspects of XML and XPath functionality in MySQL 5.5 and onwards. You may discuss these, ask questions about them, and obtain help from other users with them in the MySQL XML User Forum.

XPath expressions used with these functions support user variables and local stored program variables. User variables are weakly checked; variables local to stored programs are strongly checked (see also Bug #26518):

  • User variables (weak checking).  Variables using the syntax $@variable_name (that is, user variables) are not checked. No warnings or errors are issued by the server if a variable has the wrong type or has previously not been assigned a value. This also means the user is fully responsible for any typographical errors, since no warnings will be given if (for example) $@myvariable is used where $@myvariable was intended.

    Example:

    mysql> SET @xml = '<a><b>X</b><b>Y</b></a>';
    Query OK, 0 rows affected (0.00 sec)
    
    mysql> SET @i =1, @j = 2;
    Query OK, 0 rows affected (0.00 sec)
    
    mysql> SELECT @i, ExtractValue(@xml, '//b[$@i]');
    +------+--------------------------------+
    | @i   | ExtractValue(@xml, '//b[$@i]') |
    +------+--------------------------------+
    |    1 | X                              |
    +------+--------------------------------+
    1 row in set (0.00 sec)
    
    mysql> SELECT @j, ExtractValue(@xml, '//b[$@j]');
    +------+--------------------------------+
    | @j   | ExtractValue(@xml, '//b[$@j]') |
    +------+--------------------------------+
    |    2 | Y                              |
    +------+--------------------------------+
    1 row in set (0.00 sec)
    
    mysql> SELECT @k, ExtractValue(@xml, '//b[$@k]');
    +------+--------------------------------+
    | @k   | ExtractValue(@xml, '//b[$@k]') |
    +------+--------------------------------+
    | NULL |                                |
    +------+--------------------------------+
    1 row in set (0.00 sec)
    
  • Variables in stored programs (strong checking).  Variables using the syntax $variable_name can be declared and used with these functions when they are called inside stored programs. Such variables are local to the stored program in which they are defined, and are strongly checked for type and value.

    Example:

    mysql> DELIMITER |
    
    mysql> CREATE PROCEDURE myproc ()
        -> BEGIN
        ->   DECLARE i INT DEFAULT 1;
        ->   DECLARE xml VARCHAR(25) DEFAULT '<a>X</a><a>Y</a><a>Z</a>';
        ->
        ->   WHILE i < 4 DO
        ->     SELECT xml, i, ExtractValue(xml, '//a[$i]');
        ->     SET i = i+1;
        ->   END WHILE;
        -> END |
    Query OK, 0 rows affected (0.01 sec)
    
    mysql> DELIMITER ;
    
    mysql> CALL myproc();
    +--------------------------+---+------------------------------+
    | xml                      | i | ExtractValue(xml, '//a[$i]') |
    +--------------------------+---+------------------------------+
    | <a>X</a><a>Y</a><a>Z</a> | 1 | X                            |
    +--------------------------+---+------------------------------+
    1 row in set (0.00 sec)
    
    +--------------------------+---+------------------------------+
    | xml                      | i | ExtractValue(xml, '//a[$i]') |
    +--------------------------+---+------------------------------+
    | <a>X</a><a>Y</a><a>Z</a> | 2 | Y                            |
    +--------------------------+---+------------------------------+
    1 row in set (0.01 sec)
    
    +--------------------------+---+------------------------------+
    | xml                      | i | ExtractValue(xml, '//a[$i]') |
    +--------------------------+---+------------------------------+
    | <a>X</a><a>Y</a><a>Z</a> | 3 | Z                            |
    +--------------------------+---+------------------------------+
    1 row in set (0.01 sec)
    

    Parameters.  Variables used in XPath expressions inside stored routines that are passed in as parameters are also subject to strong checking.

Expressions containing user variables or variables local to stored programs must otherwise (except for notation) conform to the rules for XPath expressions containing variables as given in the XPath 1.0 specification.

Note

Currently, a user variable used to store an XPath expression is treated as an empty string. Because of this, it is not possible to store an XPath expression as a user variable. (Bug #32911)

  • ExtractValue(xml_frag, xpath_expr)

    ExtractValue() takes two string arguments, a fragment of XML markup xml_frag and an XPath expression xpath_expr (also known as a locator); it returns the text (CDATA) of the first text node which is a child of the elements or elements matched by the XPath expression. In MySQL 5.5, the XPath expression can contain at most 127 characters. (This limitation is lifted in MySQL 5.6.)

    Using this function is the equivalent of performing a match using the xpath_expr after appending /text(). In other words, ExtractValue('<a><b>Sakila</b></a>', '/a/b') and ExtractValue('<a><b>Sakila</b></a>', '/a/b/text()') produce the same result.

    If multiple matches are found, the content of the first child text node of each matching element is returned (in the order matched) as a single, space-delimited string.

    If no matching text node is found for the expression (including the implicit /text())—for whatever reason, as long as xpath_expr is valid, and xml_frag consists of elements which are properly nested and closed—an empty string is returned. No distinction is made between a match on an empty element and no match at all. This is by design.

    If you need to determine whether no matching element was found in xml_frag or such an element was found but contained no child text nodes, you should test the result of an expression that uses the XPath count() function. For example, both of these statements return an empty string, as shown here:

    mysql> SELECT ExtractValue('<a><b/></a>', '/a/b');
    +-------------------------------------+
    | ExtractValue('<a><b/></a>', '/a/b') |
    +-------------------------------------+
    |                                     |
    +-------------------------------------+
    1 row in set (0.00 sec)
    
    mysql> SELECT ExtractValue('<a><c/></a>', '/a/b');
    +-------------------------------------+
    | ExtractValue('<a><c/></a>', '/a/b') |
    +-------------------------------------+
    |                                     |
    +-------------------------------------+
    1 row in set (0.00 sec)
    

    However, you can determine whether there was actually a matching element using the following:

    mysql> SELECT ExtractValue('<a><b/></a>', 'count(/a/b)');
    +-------------------------------------+
    | ExtractValue('<a><b/></a>', 'count(/a/b)') |
    +-------------------------------------+
    | 1                                   |
    +-------------------------------------+
    1 row in set (0.00 sec)
    
    mysql> SELECT ExtractValue('<a><c/></a>', 'count(/a/b)');
    +-------------------------------------+
    | ExtractValue('<a><c/></a>', 'count(/a/b)') |
    +-------------------------------------+
    | 0                                   |
    +-------------------------------------+
    1 row in set (0.01 sec)
    
    Important

    ExtractValue() returns only CDATA, and does not return any tags that might be contained within a matching tag, nor any of their content (see the result returned as val1 in the following example).

    mysql> SELECT
        ->   ExtractValue('<a>ccc<b>ddd</b></a>', '/a') AS val1,
        ->   ExtractValue('<a>ccc<b>ddd</b></a>', '/a/b') AS val2,
        ->   ExtractValue('<a>ccc<b>ddd</b></a>', '//b') AS val3,
        ->   ExtractValue('<a>ccc<b>ddd</b></a>', '/b') AS val4,
        ->   ExtractValue('<a>ccc<b>ddd</b><b>eee</b></a>', '//b') AS val5;
    
    +------+------+------+------+---------+
    | val1 | val2 | val3 | val4 | val5    |
    +------+------+------+------+---------+
    | ccc  | ddd  | ddd  |      | ddd eee |
    +------+------+------+------+---------+
    

    This function uses the current SQL collation for making comparisons with contains(), performing the same collation aggregation as other string functions (such as CONCAT()), in taking into account the collation coercibility of their arguments; see Section 10.1.7.5, “Collation of Expressions”, for an explanation of the rules governing this behavior.

    (Previously, binary—that is, case-sensitive—comparison was always used.)

    NULL is returned if xml_frag contains elements which are not properly nested or closed, and a warning is generated, as shown in this example:

    mysql> SELECT ExtractValue('<a>c</a><b', '//a');
    +-----------------------------------+
    | ExtractValue('<a>c</a><b', '//a') |
    +-----------------------------------+
    | NULL                              |
    +-----------------------------------+
    1 row in set, 1 warning (0.00 sec)
    
    mysql> SHOW WARNINGS\G
    *************************** 1. row ***************************
      Level: Warning
       Code: 1525
    Message: Incorrect XML value: 'parse error at line 1 pos 11:
             END-OF-INPUT unexpected ('>' wanted)'
    1 row in set (0.00 sec)
    
    mysql> SELECT ExtractValue('<a>c</a><b/>', '//a');
    +-------------------------------------+
    | ExtractValue('<a>c</a><b/>', '//a') |
    +-------------------------------------+
    | c                                   |
    +-------------------------------------+
    1 row in set (0.00 sec)
    
  • UpdateXML(xml_target, xpath_expr, new_xml)

    This function replaces a single portion of a given fragment of XML markup xml_target with a new XML fragment new_xml, and then returns the changed XML. The portion of xml_target that is replaced matches an XPath expression xpath_expr supplied by the user. In MySQL 5.5, the XPath expression can contain at most 127 characters. (This limitation is lifted in MySQL 5.6.)

    If no expression matching xpath_expr is found, or if multiple matches are found, the function returns the original xml_target XML fragment. All three arguments should be strings.

    mysql> SELECT
        ->   UpdateXML('<a><b>ccc</b><d></d></a>', '/a', '<e>fff</e>') AS val1,
        ->   UpdateXML('<a><b>ccc</b><d></d></a>', '/b', '<e>fff</e>') AS val2,
        ->   UpdateXML('<a><b>ccc</b><d></d></a>', '//b', '<e>fff</e>') AS val3,
        ->   UpdateXML('<a><b>ccc</b><d></d></a>', '/a/d', '<e>fff</e>') AS val4,
        ->   UpdateXML('<a><d></d><b>ccc</b><d></d></a>', '/a/d', '<e>fff</e>') AS val5
        -> \G
    
    *************************** 1. row ***************************
    val1: <e>fff</e>
    val2: <a><b>ccc</b><d></d></a>
    val3: <a><e>fff</e><d></d></a>
    val4: <a><b>ccc</b><e>fff</e></a>
    val5: <a><d></d><b>ccc</b><d></d></a>
    
Note

A discussion in depth of XPath syntax and usage are beyond the scope of this Manual. Please see the XML Path Language (XPath) 1.0 specification for definitive information. A useful resource for those new to XPath or who are wishing a refresher in the basics is the Zvon.org XPath Tutorial, which is available in several languages.

Descriptions and examples of some basic XPath expressions follow:

  • /tag

    Matches <tag/> if and only if <tag/> is the root element.

    Example: /a has a match in <a><b/></a> because it matches the outermost (root) tag. It does not match the inner a element in <b><a/></b> because in this instance it is the child of another element.

  • /tag1/tag2

    Matches <tag2/> if and only if it is a child of <tag1/>, and <tag1/> is the root element.

    Example: /a/b matches the b element in the XML fragment <a><b/></a> because it is a child of the root element a. It does not have a match in <b><a/></b> because in this case, b is the root element (and hence the child of no other element). Nor does the XPath expression have a match in <a><c><b/></c></a>; here, b is a descendant of a, but not actually a child of a.

    This construct is extendable to three or more elements. For example, the XPath expression /a/b/c matches the c element in the fragment <a><b><c/></b></a>.

  • //tag

    Matches any instance of <tag>.

    Example: //a matches the a element in any of the following: <a><b><c/></b></a>; <c><a><b/></a></b>; <c><b><a/></b></c>.

    // can be combined with /. For example, //a/b matches the b element in either of the fragments <a><b/></a> or <a><b><c/></b></a>

    Note

    //tag is the equivalent of /descendant-or-self::*/tag. A common error is to confuse this with /descendant-or-self::tag, although the latter expression can actually lead to very different results, as can be seen here:

    mysql> SET @xml = '<a><b><c>w</c><b>x</b><d>y</d>z</b></a>';
    Query OK, 0 rows affected (0.00 sec)
    
    mysql> SELECT @xml;
    +-----------------------------------------+
    | @xml                                    |
    +-----------------------------------------+
    | <a><b><c>w</c><b>x</b><d>y</d>z</b></a> |
    +-----------------------------------------+
    1 row in set (0.00 sec)
    
    mysql> SELECT ExtractValue(@xml, '//b[1]');
    +------------------------------+
    | ExtractValue(@xml, '//b[1]') |
    +------------------------------+
    | x z                          |
    +------------------------------+
    1 row in set (0.00 sec)
    
    mysql> SELECT ExtractValue(@xml, '//b[2]');
    +------------------------------+
    | ExtractValue(@xml, '//b[2]') |
    +------------------------------+
    |                              |
    +------------------------------+
    1 row in set (0.01 sec)
    
    mysql> SELECT ExtractValue(@xml, '/descendant-or-self::*/b[1]');
    +---------------------------------------------------+
    | ExtractValue(@xml, '/descendant-or-self::*/b[1]') |
    +---------------------------------------------------+
    | x z                                               |
    +---------------------------------------------------+
    1 row in set (0.06 sec)
    
    mysql> SELECT ExtractValue(@xml, '/descendant-or-self::*/b[2]');
    +---------------------------------------------------+
    | ExtractValue(@xml, '/descendant-or-self::*/b[2]') |
    +---------------------------------------------------+
    |                                                   |
    +---------------------------------------------------+
    1 row in set (0.00 sec)
    
    
    mysql> SELECT ExtractValue(@xml, '/descendant-or-self::b[1]');
    +-------------------------------------------------+
    | ExtractValue(@xml, '/descendant-or-self::b[1]') |
    +-------------------------------------------------+
    | z                                               |
    +-------------------------------------------------+
    1 row in set (0.00 sec)
    
    mysql> SELECT ExtractValue(@xml, '/descendant-or-self::b[2]');
    +-------------------------------------------------+
    | ExtractValue(@xml, '/descendant-or-self::b[2]') |
    +-------------------------------------------------+
    | x                                               |
    +-------------------------------------------------+
    1 row in set (0.00 sec)
    
  • The * operator acts as a wildcard that matches any element. For example, the expression /*/b matches the b element in either of the XML fragments <a><b/></a> or <c><b/></c>. However, the expression does not produce a match in the fragment <b><a/></b> because b must be a child of some other element. The wildcard may be used in any position: The expression /*/b/* will match any child of a b element that is itself not the root element.

  • You can match any of several locators using the | (UNION) operator. For example, the expression //b|//c matches all b and c elements in the XML target.

  • It is also possible to match an element based on the value of one or more of its attributes. This done using the syntax tag[@attribute="value"]. For example, the expression //b[@id="idB"] matches the second b element in the fragment <a><b id="idA"/><c/><b id="idB"/></a>. To match against any element having attribute="value", use the XPath expression //*[attribute="value"].

    To filter multiple attribute values, simply use multiple attribute-comparison clauses in succession. For example, the expression //b[@c="x"][@d="y"] matches the element <b c="x" d="y"/> occurring anywhere in a given XML fragment.

    To find elements for which the same attribute matches any of several values, you can use multiple locators joined by the | operator. For example, to match all b elements whose c attributes have either of the values 23 or 17, use the expression //b[@c="23"]|//b[@c="17"]. You can also use the logical or operator for this purpose: //b[@c="23" or @c="17"].

    Note

    The difference between or and | is that or joins conditions, while | joins result sets.

XPath Limitations.  The XPath syntax supported by these functions is currently subject to the following limitations:

  • Nodeset-to-nodeset comparison (such as '/a/b[@c=@d]') is not supported.

  • All of the standard XPath comparison operators are supported. (Bug #22823)

  • Relative locator expressions are resolved in the context of the root node. For example, consider the following query and result:

    mysql> SELECT ExtractValue(
        ->   '<a><b c="1">X</b><b c="2">Y</b></a>',
        ->    'a/b'
        -> ) AS result;
    +--------+
    | result |
    +--------+
    | X Y    |
    +--------+
    1 row in set (0.03 sec)
    

    In this case, the locator a/b resolves to /a/b.

    Relative locators are also supported within predicates. In the following example, d[../@c="1"] is resolved as /a/b[@c="1"]/d:

    mysql> SELECT ExtractValue(
        ->      '<a>
        ->        <b c="1"><d>X</d></b>
        ->        <b c="2"><d>X</d></b>
        ->      </a>',
        ->      'a/b/d[../@c="1"]')
        -> AS result;
    +--------+
    | result |
    +--------+
    | X      |
    +--------+
    1 row in set (0.00 sec)
    
  • Locators prefixed with expressions that evaluate as scalar values—including variable references, literals, numbers, and scalar function calls—are not permitted, and their use results in an error.

  • The :: operator is not supported in combination with node types such as the following:

    • axis::comment()

    • axis::text()

    • axis::processing-instructions()

    • axis::node()

    However, name tests (such as axis::name and axis::*) are supported, as shown in these examples:

    mysql> SELECT ExtractValue('<a><b>x</b><c>y</c></a>','/a/child::b');
    +-------------------------------------------------------+
    | ExtractValue('<a><b>x</b><c>y</c></a>','/a/child::b') |
    +-------------------------------------------------------+
    | x                                                     |
    +-------------------------------------------------------+
    1 row in set (0.02 sec)
    
    mysql> SELECT ExtractValue('<a><b>x</b><c>y</c></a>','/a/child::*');
    +-------------------------------------------------------+
    | ExtractValue('<a><b>x</b><c>y</c></a>','/a/child::*') |
    +-------------------------------------------------------+
    | x y                                                   |
    +-------------------------------------------------------+
    1 row in set (0.01 sec)
    
  • Up-and-down navigation is not supported in cases where the path would lead above the root element. That is, you cannot use expressions which match on descendants of ancestors of a given element, where one or more of the ancestors of the current element is also an ancestor of the root element (see Bug #16321).

  • The following XPath functions are not supported, or have known issues as indicated:

    • id()

    • lang()

    • local-name()

    • name()

    • namespace-uri()

    • normalize-space()

    • starts-with()

    • string()

    • substring-after()

    • substring-before()

    • translate()

  • The following axes are not supported:

    • following-sibling

    • following

    • preceding-sibling

    • preceding

XPath expressions passed as arguments to ExtractValue() and UpdateXML() may contain the colon character (:) in element selectors, which enables their use with markup employing XML namespaces notation. For example:

mysql> SET @xml = '<a>111<b:c>222<d>333</d><e:f>444</e:f></b:c></a>';
Query OK, 0 rows affected (0.00 sec)

mysql> SELECT ExtractValue(@xml, '//e:f');
+-----------------------------+
| ExtractValue(@xml, '//e:f') |
+-----------------------------+
| 444                         |
+-----------------------------+
1 row in set (0.00 sec)

mysql> SELECT UpdateXML(@xml, '//b:c', '<g:h>555</g:h>');
+--------------------------------------------+
| UpdateXML(@xml, '//b:c', '<g:h>555</g:h>') |
+--------------------------------------------+
| <a>111<g:h>555</g:h></a>                   |
+--------------------------------------------+
1 row in set (0.00 sec)

This is similar in some respects to what is permitted by Apache Xalan and some other parsers, and is much simpler than requiring namespace declarations or the use of the namespace-uri() and local-name() functions.

Error handling.  For both ExtractValue() and UpdateXML(), the XPath locator used must be valid and the XML to be searched must consist of elements which are properly nested and closed. If the locator is invalid, an error is generated:

mysql> SELECT ExtractValue('<a>c</a><b/>', '/&a');
ERROR 1105 (HY000): XPATH syntax error: '&a'

If xml_frag does not consist of elements which are properly nested and closed, NULL is returned and a warning is generated, as shown in this example:

mysql> SELECT ExtractValue('<a>c</a><b', '//a');
+-----------------------------------+
| ExtractValue('<a>c</a><b', '//a') |
+-----------------------------------+
| NULL                              |
+-----------------------------------+
1 row in set, 1 warning (0.00 sec)

mysql> SHOW WARNINGS\G
*************************** 1. row ***************************
  Level: Warning
   Code: 1525
Message: Incorrect XML value: 'parse error at line 1 pos 11:
         END-OF-INPUT unexpected ('>' wanted)'
1 row in set (0.00 sec)

mysql> SELECT ExtractValue('<a>c</a><b/>', '//a');
+-------------------------------------+
| ExtractValue('<a>c</a><b/>', '//a') |
+-------------------------------------+
| c                                   |
+-------------------------------------+
1 row in set (0.00 sec)
Important

The replacement XML used as the third argument to UpdateXML() is not checked to determine whether it consists solely of elements which are properly nested and closed.

XPath Injection.  code injection occurs when malicious code is introduced into the system to gain unauthorized access to privileges and data. It is based on exploiting assumptions made by developers about the type and content of data input from users. XPath is no exception in this regard.

A common scenario in which this can happen is the case of application which handles authorization by matching the combination of a login name and password with those found in an XML file, using an XPath expression like this one:

//user[login/text()='neapolitan' and password/text()='1c3cr34m']/attribute::id

This is the XPath equivalent of an SQL statement like this one:

SELECT id FROM users WHERE login='neapolitan' AND password='1c3cr34m';

A PHP application employing XPath might handle the login process like this:

<?php

  $file     =   "users.xml";

  $login    =   $POST["login"];
  $password =   $POST["password"];

  $xpath = "//user[login/text()=$login and password/text()=$password]/attribute::id";

  if( file_exists($file) )
  {
    $xml = simplexml_load_file($file);

    if($result = $xml->xpath($xpath))
      echo "You are now logged in as user $result[0].";
    else
      echo "Invalid login name or password.";
  }
  else
    exit("Failed to open $file.");

?>

No checks are performed on the input. This means that a malevolent user can short-circuit the test by entering ' or 1=1 for both the login name and password, resulting in $xpath being evaluated as shown here:

//user[login/text()='' or 1=1 and password/text()='' or 1=1]/attribute::id

Since the expression inside the square brackets always evaluates as true, it is effectively the same as this one, which matches the id attribute of every user element in the XML document:

//user/attribute::id

One way in which this particular attack can be circumvented is simply by quoting the variable names to be interpolated in the definition of $xpath, forcing the values passed from a Web form to be converted to strings:

$xpath = "//user[login/text()='$login' and password/text()='$password']/attribute::id";

This is the same strategy that is often recommended for preventing SQL injection attacks. In general, the practices you should follow for preventing XPath injection attacks are the same as for preventing SQL injection:

  • Never accepted untested data from users in your application.

  • Check all user-submitted data for type; reject or convert data that is of the wrong type

  • Test numeric data for out of range values; truncate, round, or reject values that are out of range. Test strings for illegal characters and either strip them out or reject input containing them.

  • Do not output explicit error messages that might provide an unauthorized user with clues that could be used to compromise the system; log these to a file or database table instead.

Just as SQL injection attacks can be used to obtain information about database schemas, so can XPath injection be used to traverse XML files to uncover their structure, as discussed in Amit Klein's paper Blind XPath Injection (PDF file, 46KB).

It is also important to check the output being sent back to the client. Consider what can happen when we use the MySQL ExtractValue() function:

mysql> SELECT ExtractValue(
    ->     LOAD_FILE('users.xml'),
    ->     '//user[login/text()="" or 1=1 and password/text()="" or 1=1]/attribute::id'
    -> ) AS id;
+-------------------------------+
| id                            |
+-------------------------------+
| 00327 13579 02403 42354 28570 |
+-------------------------------+
1 row in set (0.01 sec)

Because ExtractValue() returns multiple matches as a single space-delimited string, this injection attack provides every valid ID contained within users.xml to the user as a single row of output. As an extra safeguard, you should also test output before returning it to the user. Here is a simple example:

mysql> SELECT @id = ExtractValue(
    ->     LOAD_FILE('users.xml'),
    ->     '//user[login/text()="" or 1=1 and password/text()="" or 1=1]/attribute::id'
    -> );
Query OK, 0 rows affected (0.00 sec)

mysql> SELECT IF(
    ->     INSTR(@id, ' ') = 0,
    ->     @id,
    ->     'Unable to retrieve user ID')
    -> AS singleID;
+----------------------------+
| singleID                   |
+----------------------------+
| Unable to retrieve user ID |
+----------------------------+
1 row in set (0.00 sec)

In general, the guidelines for returning data to users securely are the same as for accepting user input. These can be summed up as:

  • Always test outgoing data for type and permissible values.

  • Never permit unauthorized users to view error messages that might provide information about the application that could be used to exploit it.

12.12 Bit Functions

Table 12.16 Bitwise Functions

NameDescription
BIT_COUNT()Return the number of bits that are set
&Bitwise AND
~Invert bits
|Bitwise OR
^Bitwise XOR
<<Left shift
>>Right shift

MySQL uses BIGINT (64-bit) arithmetic for bit operations, so these operators have a maximum range of 64 bits.

  • |

    Bitwise OR:

    mysql> SELECT 29 | 15;
            -> 31
    

    The result is an unsigned 64-bit integer.

  • &

    Bitwise AND:

    mysql> SELECT 29 & 15;
            -> 13
    

    The result is an unsigned 64-bit integer.

  • ^

    Bitwise XOR:

    mysql> SELECT 1 ^ 1;
            -> 0
    mysql> SELECT 1 ^ 0;
            -> 1
    mysql> SELECT 11 ^ 3;
            -> 8
    

    The result is an unsigned 64-bit integer.

  • <<

    Shifts a longlong (BIGINT) number to the left.

    mysql> SELECT 1 << 2;
            -> 4
    

    The result is an unsigned 64-bit integer. The value is truncated to 64 bits. In particular, if the shift count is greater or equal to the width of an unsigned 64-bit number, the result is zero.

  • >>

    Shifts a longlong (BIGINT) number to the right.

    mysql> SELECT 4 >> 2;
            -> 1
    

    The result is an unsigned 64-bit integer. The value is truncated to 64 bits. In particular, if the shift count is greater or equal to the width of an unsigned 64-bit number, the result is zero.

  • ~

    Invert all bits.

    mysql> SELECT 5 & ~1;
            -> 4
    

    The result is an unsigned 64-bit integer.

  • BIT_COUNT(N)

    Returns the number of bits that are set in the argument N.

    mysql> SELECT BIT_COUNT(29), BIT_COUNT(b'101010');
            -> 4, 3
    

12.13 Encryption and Compression Functions

Table 12.17 Encryption Functions

NameDescription
AES_DECRYPT()Decrypt using AES
AES_ENCRYPT()Encrypt using AES
COMPRESS()Return result as a binary string
DECODE()Decodes a string encrypted using ENCODE()
DES_DECRYPT() (deprecated 5.7.6)Decrypt a string
DES_ENCRYPT() (deprecated 5.7.6)Encrypt a string
ENCODE()Encode a string
ENCRYPT() (deprecated 5.7.6)Encrypt a string
MD5()Calculate MD5 checksum
OLD_PASSWORD() (deprecated 5.6.5)Return the value of the pre-4.1 implementation of PASSWORD
PASSWORD()Calculate and return a password string
SHA1(), SHA()Calculate an SHA-1 160-bit checksum
SHA2()Calculate an SHA-2 checksum
UNCOMPRESS()Uncompress a string compressed
UNCOMPRESSED_LENGTH()Return the length of a string before compression

Many encryption and compression functions return strings for which the result might contain arbitrary byte values. If you want to store these results, use a column with a VARBINARY or BLOB binary string data type. This will avoid potential problems with trailing space removal or character set conversion that would change data values, such as may occur if you use a nonbinary string data type (CHAR, VARCHAR, TEXT).

Some encryption functions return strings of ASCII characters: MD5(), OLD_PASSWORD(), PASSWORD(), SHA(), SHA1(). As of MySQL 5.5.3, their return value is a nonbinary string that has a character set and collation determined by the character_set_connection and collation_connection system variables. Before 5.5.3, these functions return binary strings. The same change was made for SHA2() in MySQL 5.5.6.

For versions in which functions such as MD5() or SHA1() return a string of hex digits as a binary string, the return value cannot be converted to uppercase or compared in case-insensitive fashion as is. You must convert the value to a nonbinary string. See the discussion of binary string conversion in Section 12.10, “Cast Functions and Operators”.

If an application stores values from a function such as MD5() or SHA1() that returns a string of hex digits, more efficient storage and comparisons can be obtained by converting the hex representation to binary using UNHEX() and storing the result in a BINARY(N) column. Each pair of hex digits requires one byte in binary form, so the value of N depends on the length of the hex string. N is 16 for an MD5() value and 20 for a SHA1() value. For SHA2(), N ranges from 28 to 32 depending on the argument specifying the desired bit length of the result.

The size penalty for storing the hex string in a CHAR column is at least two times, up to eight times if the value is stored in a column that uses the utf8 character set (where each character uses 4 bytes). Storing the string also results in slower comparisons because of the larger values and the need to take character set collation rules into account.

Suppose that an application stores MD5() string values in a CHAR(32) column:

CREATE TABLE md5_tbl (md5_val CHAR(32), ...);
INSERT INTO md5_tbl (md5_val, ...) VALUES(MD5('abcdef'), ...);

To convert hex strings to more compact form, modify the application to use UNHEX() and BINARY(16) instead as follows:

CREATE TABLE md5_tbl (md5_val BINARY(16), ...);
INSERT INTO md5_tbl (md5_val, ...) VALUES(UNHEX(MD5('abcdef')), ...);

Applications should be prepared to handle the very rare case that a hashing function produces the same value for two different input values. One way to make collisions detectable is to make the hash column a primary key.

Note

Exploits for the MD5 and SHA-1 algorithms have become known. You may wish to consider using one of the other encryption functions described in this section instead, such as SHA2().

Caution

Passwords or other sensitive values supplied as arguments to encryption functions are sent in plaintext to the MySQL server unless an SSL connection is used. Also, such values will appear in any MySQL logs to which they are written. To avoid these types of exposure, applications can encrypt sensitive values on the client side before sending them to the server. The same considerations apply to encryption keys. To avoid exposing these, applications can use stored procedures to encrypt and decrypt values on the server side.

  • AES_DECRYPT(crypt_str,key_str)

    This function decrypts data using the official AES (Advanced Encryption Standard) algorithm. For more information, see the description of AES_ENCRYPT().

  • AES_ENCRYPT(str,key_str)

    AES_ENCRYPT() and AES_DECRYPT() implement encryption and decryption of data using the official AES (Advanced Encryption Standard) algorithm, previously known as Rijndael. The AES standard permits various key lengths. These functions implement AES with a 128-bit key length, but you can extend them to 256 bits by modifying the source. The key length is a trade off between performance and security.

    AES_ENCRYPT() encrypts the string str using the key string key_str and returns a binary string containing the encrypted output. AES_DECRYPT() decrypts the encrypted string crypt_str using the key string key_str and returns the original plaintext string. If either function argument is NULL, the function returns NULL.

    The str and crypt_str arguments can be any length, and padding is automatically added to str so it is a multiple of a block as required by block-based algorithms such as AES. This padding is automatically removed by the AES_DECRYPT() function. The length of crypt_str can be calculated using this formula:

    16 * (trunc(string_length / 16) + 1)
    

    For a key length of 128 bits, the most secure way to pass a key to the key_str argument is to create a truly random 128-bit value and pass it as a binary value. For example:

    INSERT INTO t
    VALUES (1,AES_ENCRYPT('text',UNHEX('F3229A0B371ED2D9441B830D21A390C3')));
    

    A passphrase can be used to generate an AES key by hashing the passphrase. For example:

    INSERT INTO t VALUES (1,AES_ENCRYPT('text', SHA2('My secret passphrase',512)));
    

    Do not pass a password or passphrase directly to crypt_str, hash it first. Previous versions of this documentation suggested the former approach, but it is no longer recommended as the examples shown here are more secure.

    If AES_DECRYPT() detects invalid data or incorrect padding, it returns NULL. However, it is possible for AES_DECRYPT() to return a non-NULL value (possibly garbage) if the input data or the key is invalid.

  • COMPRESS(string_to_compress)

    Compresses a string and returns the result as a binary string. This function requires MySQL to have been compiled with a compression library such as zlib. Otherwise, the return value is always NULL. The compressed string can be uncompressed with UNCOMPRESS().

    mysql> SELECT LENGTH(COMPRESS(REPEAT('a',1000)));
            -> 21
    mysql> SELECT LENGTH(COMPRESS(''));
            -> 0
    mysql> SELECT LENGTH(COMPRESS('a'));
            -> 13
    mysql> SELECT LENGTH(COMPRESS(REPEAT('a',16)));
            -> 15
    

    The compressed string contents are stored the following way:

    • Empty strings are stored as empty strings.

    • Nonempty strings are stored as a 4-byte length of the uncompressed string (low byte first), followed by the compressed string. If the string ends with space, an extra . character is added to avoid problems with endspace trimming should the result be stored in a CHAR or VARCHAR column. (However, use of nonbinary string data types such as CHAR or VARCHAR to store compressed strings is not recommended anyway because character set conversion may occur. Use a VARBINARY or BLOB binary string column instead.)

  • DECODE(crypt_str,pass_str)

    Decrypts the encrypted string crypt_str using pass_str as the password. crypt_str should be a string returned from ENCODE().

  • DES_DECRYPT(crypt_str[,key_str])

    Decrypts a string encrypted with DES_ENCRYPT(). If an error occurs, this function returns NULL.

    This function works only if MySQL has been configured with SSL support. See Section 6.3.9, “Using SSL for Secure Connections”.

    If no key_str argument is given, DES_DECRYPT() examines the first byte of the encrypted string to determine the DES key number that was used to encrypt the original string, and then reads the key from the DES key file to decrypt the message. For this to work, the user must have the SUPER privilege. The key file can be specified with the --des-key-file server option.

    If you pass this function a key_str argument, that string is used as the key for decrypting the message.

    If the crypt_str argument does not appear to be an encrypted string, MySQL returns the given crypt_str.

  • DES_ENCRYPT(str[,{key_num|key_str}])

    Encrypts the string with the given key using the Triple-DES algorithm.

    This function works only if MySQL has been configured with SSL support. See Section 6.3.9, “Using SSL for Secure Connections”.

    The encryption key to use is chosen based on the second argument to DES_ENCRYPT(), if one was given. With no argument, the first key from the DES key file is used. With a key_num argument, the given key number (0 to 9) from the DES key file is used. With a key_str argument, the given key string is used to encrypt str.

    The key file can be specified with the --des-key-file server option.

    The return string is a binary string where the first character is CHAR(128 | key_num). If an error occurs, DES_ENCRYPT() returns NULL.

    The 128 is added to make it easier to recognize an encrypted key. If you use a string key, key_num is 127.

    The string length for the result is given by this formula:

    new_len = orig_len + (8 - (orig_len % 8)) + 1
    

    Each line in the DES key file has the following format:

    key_num des_key_str
    

    Each key_num value must be a number in the range from 0 to 9. Lines in the file may be in any order. des_key_str is the string that is used to encrypt the message. There should be at least one space between the number and the key. The first key is the default key that is used if you do not specify any key argument to DES_ENCRYPT().

    You can tell MySQL to read new key values from the key file with the FLUSH DES_KEY_FILE statement. This requires the RELOAD privilege.

    One benefit of having a set of default keys is that it gives applications a way to check for the existence of encrypted column values, without giving the end user the right to decrypt those values.

    mysql> SELECT customer_address FROM customer_table 
         > WHERE crypted_credit_card = DES_ENCRYPT('credit_card_number');
    
  • ENCODE(str,pass_str)

    Encrypt str using pass_str as the password. The result is a binary string of the same length as str. To decrypt the result, use DECODE().

    The ENCODE() function should no longer be used. If you still need to use ENCODE(), a salt value must be used with it to reduce risk. For example:

    ENCODE('plaintext', CONCAT('my_random_salt','my_secret_password'))
    

    A new random salt value must be used whenever a password is updated.

  • ENCRYPT(str[,salt])

    Encrypts str using the Unix crypt() system call and returns a binary string. The salt argument must be a string with at least two characters or the result will be NULL. If no salt argument is given, a random value is used.

    mysql> SELECT ENCRYPT('hello');
            -> 'VxuFAJXVARROc'
    

    ENCRYPT() ignores all but the first eight characters of str, at least on some systems. This behavior is determined by the implementation of the underlying crypt() system call.

    The use of ENCRYPT() with the ucs2, utf16, or utf32 multibyte character sets is not recommended because the system call expects a string terminated by a zero byte.

    If crypt() is not available on your system (as is the case with Windows), ENCRYPT() always returns NULL.

  • MD5(str)

    Calculates an MD5 128-bit checksum for the string. The value is returned as a string of 32 hex digits, or NULL if the argument was NULL. The return value can, for example, be used as a hash key. See the notes at the beginning of this section about storing hash values efficiently.

    As of MySQL 5.5.3, the return value is a nonbinary string in the connection character set. Before 5.5.3, the return value is a binary string; see the notes at the beginning of this section about using the value as a nonbinary string.

    mysql> SELECT MD5('testing');
            -> 'ae2b1fca515949e5d54fb22b8ed95575'
    

    This is the RSA Data Security, Inc. MD5 Message-Digest Algorithm.

    See the note regarding the MD5 algorithm at the beginning this section.

  • OLD_PASSWORD(str)

    OLD_PASSWORD() was added when the implementation of PASSWORD() was changed in MySQL 4.1 to improve security. OLD_PASSWORD() returns the value of the pre-4.1 implementation of PASSWORD() as a string, and is intended to permit you to reset passwords for any pre-4.1 clients that need to connect to your version 5.5 MySQL server without locking them out. See Section 6.1.2.4, “Password Hashing in MySQL”.

    As of MySQL 5.5.3, the return value is a nonbinary string in the connection character set. Before 5.5.3, the return value is a binary string.

    Note

    Passwords that use the pre-4.1 hashing method are less secure than passwords that use the native password hashing method and should be avoided.

  • PASSWORD(str)

    Returns a hashed password string calculated from the cleartext password str. The return value is a nonbinary string in the connection character set (a binary string before MySQL 5.5.3), or NULL if the argument is NULL. This function is the SQL interface to the algorithm used by the server to encrypt MySQL passwords for storage in the mysql.user grant table.

    The old_passwords system variable controls the password hashing method used by the PASSWORD() function. It also influences password hashing performed by CREATE USER and GRANT statements that specify a password using an IDENTIFIED BY clause.

    The following table shows the permitted values of old_passwords, the password hashing method for each value, and which authentication plugins use passwords hashed with each method.

    ValuePassword Hashing MethodAssociated Authentication Plugin
    0 or OFFMySQL 4.1 native hashingmysql_native_password
    1 or ONPre-4.1 (old) hashingmysql_old_password

    If old_passwords=1, PASSWORD(str) returns the same value as OLD_PASSWORD(str). The latter function is not affected by the value of old_passwords.

    mysql> SET old_passwords = 0;
    mysql> SELECT PASSWORD('mypass'), OLD_PASSWORD('mypass');
    +-------------------------------------------+------------------------+
    | PASSWORD('mypass')                        | OLD_PASSWORD('mypass') |
    +-------------------------------------------+------------------------+
    | *6C8989366EAF75BB670AD8EA7A7FC1176A95CEF4 | 6f8c114b58f2ce9e       |
    +-------------------------------------------+------------------------+
    
    mysql> SET old_passwords = 1;
    mysql> SELECT PASSWORD('mypass'), OLD_PASSWORD('mypass');
    +--------------------+------------------------+
    | PASSWORD('mypass') | OLD_PASSWORD('mypass') |
    +--------------------+------------------------+
    | 6f8c114b58f2ce9e   | 6f8c114b58f2ce9e       |
    +--------------------+------------------------+
    

    Encryption performed by PASSWORD() is one-way (not reversible). It is not the same type of encryption as used for Unix passwords; for that, use ENCRYPT().

    Note

    The PASSWORD() function is used by the authentication system in MySQL Server; you should not use it in your own applications. For that purpose, consider MD5() or SHA2() instead. Also see RFC 2195, section 2 (Challenge-Response Authentication Mechanism (CRAM)), for more information about handling passwords and authentication securely in your applications.

    Note

    Passwords that use the pre-4.1 hashing method are less secure than passwords that use the native password hashing method and should be avoided.

    Caution

    Statements that invoke PASSWORD() may be recorded in server logs or on the client side in a history file such as ~/.mysql_history, which means that cleartext passwords may be read by anyone having read access to that information. For information about password logging in the server logs, see Section 6.1.2.3, “Passwords and Logging”. For similar information about client-side logging, see Section 4.5.1.3, “mysql Logging”.

  • SHA1(str), SHA(str)

    Calculates an SHA-1 160-bit checksum for the string, as described in RFC 3174 (Secure Hash Algorithm). The value is returned as a string of 40 hex digits, or NULL if the argument was NULL. One of the possible uses for this function is as a hash key. See the notes at the beginning of this section about storing hash values efficiently. You can also use SHA1() as a cryptographic function for storing passwords. SHA() is synonymous with SHA1().

    As of MySQL 5.5.3, the return value is a nonbinary string in the connection character set. Before 5.5.3, the return value is a binary string; see the notes at the beginning of this section about using the value as a nonbinary string.

    mysql> SELECT SHA1('abc');
            -> 'a9993e364706816aba3e25717850c26c9cd0d89d'
    

    SHA1() can be considered a cryptographically more secure equivalent of MD5(). However, see the note regarding the MD5 and SHA-1 algorithms at the beginning this section.

  • SHA2(str, hash_length)

    Calculates the SHA-2 family of hash functions (SHA-224, SHA-256, SHA-384, and SHA-512). The first argument is the cleartext string to be hashed. The second argument indicates the desired bit length of the result, which must have a value of 224, 256, 384, 512, or 0 (which is equivalent to 256). If either argument is NULL or the hash length is not one of the permitted values, the return value is NULL. Otherwise, the function result is a hash value containing the desired number of bits. See the notes at the beginning of this section about storing hash values efficiently.

    As of MySQL 5.5.6, the return value is a nonbinary string in the connection character set. Before 5.5.6, the return value is a binary string; see the notes at the beginning of this section about using the value as a nonbinary string.

    mysql> SELECT SHA2('abc', 224);
            -> '23097d223405d8228642a477bda255b32aadbce4bda0b3f7e36c9da7'
    

    This function works only if MySQL has been configured with SSL support. See Section 6.3.9, “Using SSL for Secure Connections”.

    SHA2() can be considered cryptographically more secure than MD5() or SHA1().

    SHA2() was added in MySQL 5.5.5.

  • UNCOMPRESS(string_to_uncompress)

    Uncompresses a string compressed by the COMPRESS() function. If the argument is not a compressed value, the result is NULL. This function requires MySQL to have been compiled with a compression library such as zlib. Otherwise, the return value is always NULL.

    mysql> SELECT UNCOMPRESS(COMPRESS('any string'));
            -> 'any string'
    mysql> SELECT UNCOMPRESS('any string');
            -> NULL
    
  • UNCOMPRESSED_LENGTH(compressed_string)

    Returns the length that the compressed string had before being compressed.

    mysql> SELECT UNCOMPRESSED_LENGTH(COMPRESS(REPEAT('a',30)));
            -> 30
    

12.14 Information Functions

Table 12.18 Information Functions

NameDescription
BENCHMARK()Repeatedly execute an expression
CHARSET()Return the character set of the argument
COERCIBILITY()Return the collation coercibility value of the string argument
COLLATION()Return the collation of the string argument
CONNECTION_ID()Return the connection ID (thread ID) for the connection
CURRENT_USER(), CURRENT_USERThe authenticated user name and host name
DATABASE()Return the default (current) database name
FOUND_ROWS()For a SELECT with a LIMIT clause, the number of rows that would be returned were there no LIMIT clause
LAST_INSERT_ID()Value of the AUTOINCREMENT column for the last INSERT
ROW_COUNT()The number of rows updated
SCHEMA()Synonym for DATABASE()
SESSION_USER()Synonym for USER()
SYSTEM_USER()Synonym for USER()
USER()The user name and host name provided by the client
VERSION()Return a string that indicates the MySQL server version

  • BENCHMARK(count,expr)

    The BENCHMARK() function executes the expression expr repeatedly count times. It may be used to time how quickly MySQL processes the expression. The result value is always 0. The intended use is from within the mysql client, which reports query execution times:

    mysql> SELECT BENCHMARK(1000000,ENCODE('hello','goodbye'));
    +----------------------------------------------+
    | BENCHMARK(1000000,ENCODE('hello','goodbye')) |
    +----------------------------------------------+
    |                                            0 |
    +----------------------------------------------+
    1 row in set (4.74 sec)
    

    The time reported is elapsed time on the client end, not CPU time on the server end. It is advisable to execute BENCHMARK() several times, and to interpret the result with regard to how heavily loaded the server machine is.

    BENCHMARK() is intended for measuring the runtime performance of scalar expressions, which has some significant implications for the way that you use it and interpret the results:

    • Only scalar expressions can be used. Although the expression can be a subquery, it must return a single column and at most a single row. For example, BENCHMARK(10, (SELECT * FROM t)) will fail if the table t has more than one column or more than one row.

    • Executing a SELECT expr statement N times differs from executing SELECT BENCHMARK(N, expr) in terms of the amount of overhead involved. The two have very different execution profiles and you should not expect them to take the same amount of time. The former involves the parser, optimizer, table locking, and runtime evaluation N times each. The latter involves only runtime evaluation N times, and all the other components just once. Memory structures already allocated are reused, and runtime optimizations such as local caching of results already evaluated for aggregate functions can alter the results. Use of BENCHMARK() thus measures performance of the runtime component by giving more weight to that component and removing the noise introduced by the network, parser, optimizer, and so forth.

  • CHARSET(str)

    Returns the character set of the string argument.

    mysql> SELECT CHARSET('abc');
            -> 'latin1'
    mysql> SELECT CHARSET(CONVERT('abc' USING utf8));
            -> 'utf8'
    mysql> SELECT CHARSET(USER());
            -> 'utf8'
    
  • COERCIBILITY(str)

    Returns the collation coercibility value of the string argument.

    mysql> SELECT COERCIBILITY('abc' COLLATE latin1_swedish_ci);
            -> 0
    mysql> SELECT COERCIBILITY(USER());
            -> 3
    mysql> SELECT COERCIBILITY('abc');
            -> 4
    

    The return values have the meanings shown in the following table. Lower values have higher precedence.

    CoercibilityMeaningExample
    0Explicit collationValue with COLLATE clause
    1No collationConcatenation of strings with different collations
    2Implicit collationColumn value, stored routine parameter or local variable
    3System constantUSER() return value
    4CoercibleLiteral string
    5IgnorableNULL or an expression derived from NULL
  • COLLATION(str)

    Returns the collation of the string argument.

    mysql> SELECT COLLATION('abc');
            -> 'latin1_swedish_ci'
    mysql> SELECT COLLATION(_utf8'abc');
            -> 'utf8_general_ci'
    
  • CONNECTION_ID()

    Returns the connection ID (thread ID) for the connection. Every connection has an ID that is unique among the set of currently connected clients.

    The value returned by CONNECTION_ID() is the same type of value as displayed in the ID column of the INFORMATION_SCHEMA.PROCESSLIST table, the Id column of SHOW PROCESSLIST output, and the PROCESSLIST_ID column of the Performance Schema threads table.

    mysql> SELECT CONNECTION_ID();
            -> 23786
    
  • CURRENT_USER, CURRENT_USER()

    Returns the user name and host name combination for the MySQL account that the server used to authenticate the current client. This account determines your access privileges. The return value is a string in the utf8 character set.

    The value of CURRENT_USER() can differ from the value of USER().

    mysql> SELECT USER();
            -> 'davida@localhost'
    mysql> SELECT * FROM mysql.user;
    ERROR 1044: Access denied for user ''@'localhost' to
    database 'mysql'
    mysql> SELECT CURRENT_USER();
            -> '@localhost'
    

    The example illustrates that although the client specified a user name of davida (as indicated by the value of the USER() function), the server authenticated the client using an anonymous user account (as seen by the empty user name part of the CURRENT_USER() value). One way this might occur is that there is no account listed in the grant tables for davida.

    Within a stored program or view, CURRENT_USER() returns the account for the user who defined the object (as given by its DEFINER value) unless defined with the SQL SECURITY INVOKER characteristic. In the latter case, CURRENT_USER() returns the object's invoker.

    Triggers and events have no option to define the SQL SECURITY characteristic, so for these objects, CURRENT_USER() returns the account for the user who defined the object. To return the invoker, use USER() or SESSION_USER().

    The following statements support use of the CURRENT_USER() function to take the place of the name of (and, possibly, a host for) an affected user or a definer; in such cases, CURRENT_USER() is expanded where and as needed:

    For information about the implications that this expansion of CURRENT_USER() has for replication in different releases of MySQL 5.5, see Section 17.4.1.7, “Replication of CURRENT_USER()”.

  • DATABASE()

    Returns the default (current) database name as a string in the utf8 character set. If there is no default database, DATABASE() returns NULL. Within a stored routine, the default database is the database that the routine is associated with, which is not necessarily the same as the database that is the default in the calling context.

    mysql> SELECT DATABASE();
            -> 'test'
    

    If there is no default database, DATABASE() returns NULL.

  • FOUND_ROWS()

    A SELECT statement may include a LIMIT clause to restrict the number of rows the server returns to the client. In some cases, it is desirable to know how many rows the statement would have returned without the LIMIT, but without running the statement again. To obtain this row count, include a SQL_CALC_FOUND_ROWS option in the SELECT statement, and then invoke FOUND_ROWS() afterward:

    mysql> SELECT SQL_CALC_FOUND_ROWS * FROM tbl_name
        -> WHERE id > 100 LIMIT 10;
    mysql> SELECT FOUND_ROWS();
    

    The second SELECT returns a number indicating how many rows the first SELECT would have returned had it been written without the LIMIT clause.

    In the absence of the SQL_CALC_FOUND_ROWS option in the most recent successful SELECT statement, FOUND_ROWS() returns the number of rows in the result set returned by that statement. If the statement includes a LIMIT clause, FOUND_ROWS() returns the number of rows up to the limit. For example, FOUND_ROWS() returns 10 or 60, respectively, if the statement includes LIMIT 10 or LIMIT 50, 10.

    The row count available through FOUND_ROWS() is transient and not intended to be available past the statement following the SELECT SQL_CALC_FOUND_ROWS statement. If you need to refer to the value later, save it:

    mysql> SELECT SQL_CALC_FOUND_ROWS * FROM ... ;
    mysql> SET @rows = FOUND_ROWS();
    

    If you are using SELECT SQL_CALC_FOUND_ROWS, MySQL must calculate how many rows are in the full result set. However, this is faster than running the query again without LIMIT, because the result set need not be sent to the client.

    SQL_CALC_FOUND_ROWS and FOUND_ROWS() can be useful in situations when you want to restrict the number of rows that a query returns, but also determine the number of rows in the full result set without running the query again. An example is a Web script that presents a paged display containing links to the pages that show other sections of a search result. Using FOUND_ROWS() enables you to determine how many other pages are needed for the rest of the result.

    The use of SQL_CALC_FOUND_ROWS and FOUND_ROWS() is more complex for UNION statements than for simple SELECT statements, because LIMIT may occur at multiple places in a UNION. It may be applied to individual SELECT statements in the UNION, or global to the UNION result as a whole.

    The intent of SQL_CALC_FOUND_ROWS for UNION is that it should return the row count that would be returned without a global LIMIT. The conditions for use of SQL_CALC_FOUND_ROWS with UNION are:

    • The SQL_CALC_FOUND_ROWS keyword must appear in the first SELECT of the UNION.

    • The value of FOUND_ROWS() is exact only if UNION ALL is used. If UNION without ALL is used, duplicate removal occurs and the value of FOUND_ROWS() is only approximate.

    • If no LIMIT is present in the UNION, SQL_CALC_FOUND_ROWS is ignored and returns the number of rows in the temporary table that is created to process the UNION.

    Beyond the cases described here, the behavior of FOUND_ROWS() is undefined (for example, its value following a SELECT statement that fails with an error).

    Important

    FOUND_ROWS() is not replicated reliably using statement-based replication. This function is automatically replicated using row-based replication.

  • LAST_INSERT_ID(), LAST_INSERT_ID(expr)

    With no argument, LAST_INSERT_ID() returns a 64-bit value representing the first automatically generated value successfully inserted for an AUTO_INCREMENT column as a result of the most recently executed INSERT statement. The value has a type of BIGINT UNSIGNED as of MySQL 5.5.29, BIGINT (signed) before that. The value of LAST_INSERT_ID() remains unchanged if no rows are successfully inserted.

    With an argument, LAST_INSERT_ID() returns an unsigned integer as of MySQL 5.5.29, a signed integer before that.

    For example, after inserting a row that generates an AUTO_INCREMENT value, you can get the value like this:

    mysql> SELECT LAST_INSERT_ID();
            -> 195
    

    The currently executing statement does not affect the value of LAST_INSERT_ID(). Suppose that you generate an AUTO_INCREMENT value with one statement, and then refer to LAST_INSERT_ID() in a multiple-row INSERT statement that inserts rows into a table with its own AUTO_INCREMENT column. The value of LAST_INSERT_ID() will remain stable in the second statement; its value for the second and later rows is not affected by the earlier row insertions. (However, if you mix references to LAST_INSERT_ID() and LAST_INSERT_ID(expr), the effect is undefined.)

    If the previous statement returned an error, the value of LAST_INSERT_ID() is undefined. For transactional tables, if the statement is rolled back due to an error, the value of LAST_INSERT_ID() is left undefined. For manual ROLLBACK, the value of LAST_INSERT_ID() is not restored to that before the transaction; it remains as it was at the point of the ROLLBACK.

    Prior to MySQL 5.5.35, this function was not replicated correctly if replication filtering rules were in use. (Bug #17234370, Bug #69861)

    Within the body of a stored routine (procedure or function) or a trigger, the value of LAST_INSERT_ID() changes the same way as for statements executed outside the body of these kinds of objects. The effect of a stored routine or trigger upon the value of LAST_INSERT_ID() that is seen by following statements depends on the kind of routine:

    • If a stored procedure executes statements that change the value of LAST_INSERT_ID(), the changed value is seen by statements that follow the procedure call.

    • For stored functions and triggers that change the value, the value is restored when the function or trigger ends, so following statements will not see a changed value.

    The ID that was generated is maintained in the server on a per-connection basis. This means that the value returned by the function to a given client is the first AUTO_INCREMENT value generated for most recent statement affecting an AUTO_INCREMENT column by that client. This value cannot be affected by other clients, even if they generate AUTO_INCREMENT values of their own. This behavior ensures that each client can retrieve its own ID without concern for the activity of other clients, and without the need for locks or transactions.

    The value of LAST_INSERT_ID() is not changed if you set the AUTO_INCREMENT column of a row to a non-magic value (that is, a value that is not NULL and not 0).

    Important

    If you insert multiple rows using a single INSERT statement, LAST_INSERT_ID() returns the value generated for the first inserted row only. The reason for this is to make it possible to reproduce easily the same INSERT statement against some other server.

    For example:

    mysql> USE test;
    Database changed
    mysql> CREATE TABLE t (
        ->   id INT AUTO_INCREMENT NOT NULL PRIMARY KEY,
        ->   name VARCHAR(10) NOT NULL
        -> );
    Query OK, 0 rows affected (0.09 sec)
    
    mysql> INSERT INTO t VALUES (NULL, 'Bob');
    Query OK, 1 row affected (0.01 sec)
    
    mysql> SELECT * FROM t;
    +----+------+
    | id | name |
    +----+------+
    |  1 | Bob  |
    +----+------+
    1 row in set (0.01 sec)
    
    mysql> SELECT LAST_INSERT_ID();
    +------------------+
    | LAST_INSERT_ID() |
    +------------------+
    |                1 |
    +------------------+
    1 row in set (0.00 sec)
    
    mysql> INSERT INTO t VALUES
        -> (NULL, 'Mary'), (NULL, 'Jane'), (NULL, 'Lisa');
    Query OK, 3 rows affected (0.00 sec)
    Records: 3  Duplicates: 0  Warnings: 0
    
    mysql> SELECT * FROM t;
    +----+------+
    | id | name |
    +----+------+
    |  1 | Bob  |
    |  2 | Mary |
    |  3 | Jane |
    |  4 | Lisa |
    +----+------+
    4 rows in set (0.01 sec)
    
    mysql> SELECT LAST_INSERT_ID();
    +------------------+
    | LAST_INSERT_ID() |
    +------------------+
    |                2 |
    +------------------+
    1 row in set (0.00 sec)
    

    Although the second INSERT statement inserted three new rows into t, the ID generated for the first of these rows was 2, and it is this value that is returned by LAST_INSERT_ID() for the following SELECT statement.

    If you use INSERT IGNORE and the row is ignored, the LAST_INSERT_ID() remains unchanged from the current value (or 0 is returned if the connection has not yet performed a successful INSERT) and, for non-transactional tables, the AUTO_INCREMENT counter is not incremented. For InnoDB tables, the AUTO_INCREMENT counter is incremented if innodb_autoinc_lock_mode is set to 1 or 2, as demonstrated in the following example:

    mysql> USE test;
    Database changed
    
    mysql> SELECT @@innodb_autoinc_lock_mode;
    +----------------------------+
    | @@innodb_autoinc_lock_mode |
    +----------------------------+
    |                          1 |
    +----------------------------+
    1 row in set (0.00 sec)
    
    mysql> CREATE TABLE `t` (
    `id` INT(11) NOT NULL AUTO_INCREMENT,
    `val` INT(11) DEFAULT NULL,
    PRIMARY KEY (`id`),
    UNIQUE KEY `i1` (`val`)
    ) ENGINE=InnoDB DEFAULT CHARSET=latin1;
    Query OK, 0 rows affected (0.02 sec)
    
    -- Insert two rows
    
    mysql> INSERT INTO t (val) VALUES (1),(2);
    Query OK, 2 rows affected (0.00 sec)
    Records: 2  Duplicates: 0  Warnings: 0
    
    -- With auto_increment_offset=1, the inserted rows
    -- result in an AUTO_INCREMENT value of 3
    
    mysql> SHOW CREATE TABLE t\G
    *************************** 1. row ***************************
           Table: t
    Create Table: CREATE TABLE `t` (
      `id` int(11) NOT NULL AUTO_INCREMENT,
      `val` int(11) DEFAULT NULL,
      PRIMARY KEY (`id`),
      UNIQUE KEY `i1` (`val`)
    ) ENGINE=MyISAM AUTO_INCREMENT=3 DEFAULT CHARSET=latin1
    1 row in set (0.00 sec)
    
    -- LAST_INSERT_ID() returns the first automatically generated 
    -- value that is successfully inserted for the AUTO_INCREMENT column
    
    mysql> SELECT LAST_INSERT_ID();
    +------------------+
    | LAST_INSERT_ID() |
    +------------------+
    |                1 |
    +------------------+
    1 row in set (0.00 sec)
    
    -- The attempted insertion of duplicate rows fail but errors are ignored     
    
    mysql> INSERT IGNORE INTO t (val) VALUES (1),(2);
    Query OK, 0 rows affected (0.00 sec)
    Records: 2  Duplicates: 2  Warnings: 0
    
    -- With innodb_autoinc_lock_mode=1, the AUTO_INCREMENT counter 
    -- is incremented for the ignored rows
    
    mysql> SHOW CREATE TABLE t\G
    *************************** 1. row ***************************
           Table: t
    Create Table: CREATE TABLE `t` (
      `id` int(11) NOT NULL AUTO_INCREMENT,
      `val` int(11) DEFAULT NULL,
      PRIMARY KEY (`id`),
      UNIQUE KEY `i1` (`val`)
    ) ENGINE=MyISAM AUTO_INCREMENT=5 DEFAULT CHARSET=latin1
    1 row in set (0.00 sec)
    
    -- The LAST_INSERT_ID is unchanged becuase the previous insert was unsuccessful
    
    mysql> SELECT LAST_INSERT_ID();
    +------------------+
    | LAST_INSERT_ID() |
    +------------------+
    |                1 |
    +------------------+
    1 row in set (0.00 sec)        

    See Section 14.9.5, “AUTO_INCREMENT Handling in InnoDB” for more information.

    If expr is given as an argument to LAST_INSERT_ID(), the value of the argument is returned by the function and is remembered as the next value to be returned by LAST_INSERT_ID(). This can be used to simulate sequences:

    1. Create a table to hold the sequence counter and initialize it:

      mysql> CREATE TABLE sequence (id INT NOT NULL);
      mysql> INSERT INTO sequence VALUES (0);
      
    2. Use the table to generate sequence numbers like this:

      mysql> UPDATE sequence SET id=LAST_INSERT_ID(id+1);
      mysql> SELECT LAST_INSERT_ID();
      

      The UPDATE statement increments the sequence counter and causes the next call to LAST_INSERT_ID() to return the updated value. The SELECT statement retrieves that value. The mysql_insert_id() C API function can also be used to get the value. See Section 23.8.7.37, “mysql_insert_id()”.

    You can generate sequences without calling LAST_INSERT_ID(), but the utility of using the function this way is that the ID value is maintained in the server as the last automatically generated value. It is multi-user safe because multiple clients can issue the UPDATE statement and get their own sequence value with the SELECT statement (or mysql_insert_id()), without affecting or being affected by other clients that generate their own sequence values.

    Note that mysql_insert_id() is only updated after INSERT and UPDATE statements, so you cannot use the C API function to retrieve the value for LAST_INSERT_ID(expr) after executing other SQL statements like SELECT or SET.

  • ROW_COUNT()

    Before MySQL 5.5.5, ROW_COUNT() returns the number of rows changed, deleted, or inserted by the last statement if it was an UPDATE, DELETE, or INSERT. For other statements, the value may not be meaningful.

    As of MySQL 5.5.5, ROW_COUNT() returns a value as follows:

    • DDL statements: 0. This applies to statements such as CREATE TABLE or DROP TABLE.

    • DML statements other than SELECT: The number of affected rows. This applies to statements such as UPDATE, INSERT, or DELETE (as before), but now also to statements such as ALTER TABLE and LOAD DATA INFILE.

    • SELECT: -1 if the statement returns a result set, or the number of rows affected if it does not. For example, for SELECT * FROM t1, ROW_COUNT() returns -1. For SELECT * FROM t1 INTO OUTFILE 'file_name', ROW_COUNT() returns the number of rows written to the file.

    • SIGNAL statements: 0.

    For UPDATE statements, the affected-rows value by default is the number of rows actually changed. If you specify the CLIENT_FOUND_ROWS flag to mysql_real_connect() when connecting to mysqld, the affected-rows value is the number of rows found; that is, matched by the WHERE clause.

    For REPLACE statements, the affected-rows value is 2 if the new row replaced an old row, because in this case, one row was inserted after the duplicate was deleted.

    For INSERT ... ON DUPLICATE KEY UPDATE statements, the affected-rows value per row is 1 if the row is inserted as a new row, 2 if an existing row is updated, and 0 if an existing row is set to its current values. If you specify the CLIENT_FOUND_ROWS flag, the affected-rows value is 1 (not 0) if an existing row is set to its current values.

    The ROW_COUNT() value is similar to the value from the mysql_affected_rows() C API function and the row count that the mysql client displays following statement execution.

    mysql> INSERT INTO t VALUES(1),(2),(3);
    Query OK, 3 rows affected (0.00 sec)
    Records: 3  Duplicates: 0  Warnings: 0
    
    mysql> SELECT ROW_COUNT();
    +-------------+
    | ROW_COUNT() |
    +-------------+
    |           3 |
    +-------------+
    1 row in set (0.00 sec)
    
    mysql> DELETE FROM t WHERE i IN(1,2);
    Query OK, 2 rows affected (0.00 sec)
    
    mysql> SELECT ROW_COUNT();
    +-------------+
    | ROW_COUNT() |
    +-------------+
    |           2 |
    +-------------+
    1 row in set (0.00 sec)
    
    Important

    ROW_COUNT() is not replicated reliably using statement-based replication. This function is automatically replicated using row-based replication.

  • SCHEMA()

    This function is a synonym for DATABASE().

  • SESSION_USER()

    SESSION_USER() is a synonym for USER().

  • SYSTEM_USER()

    SYSTEM_USER() is a synonym for USER().

  • USER()

    Returns the current MySQL user name and host name as a string in the utf8 character set.

    mysql> SELECT USER();
            -> 'davida@localhost'
    

    The value indicates the user name you specified when connecting to the server, and the client host from which you connected. The value can be different from that of CURRENT_USER().

    You can extract only the user name part like this:

    mysql> SELECT SUBSTRING_INDEX(USER(),'@',1);
            -> 'davida'
    
  • VERSION()

    Returns a string that indicates the MySQL server version. The string uses the utf8 character set. The value might have a suffix in addition to the version number. See the description of the version system variable in Section 5.1.4, “Server System Variables”.

    This function is unsafe for statement-based replication. Beginning with MySQL 5.5.1, a warning is logged if you use this function when binlog_format is set to STATEMENT. (Bug #47995)

    mysql> SELECT VERSION();
            -> '5.5.42-standard'
    

12.15 Spatial Analysis Functions

MySQL provides functions to perform various operations on spatial data. These functions can be grouped into several major categories according to the type of operation they perform:

  • Functions that create geometries in various formats (WKT, WKB, internal)

  • Functions that convert geometries between formats

  • Functions that access qualitative or quantitative properties of a geometry

  • Functions that describe relations between two geometries

  • Functions that create new geometries from existing ones

For general background about MySQL support for using spatial data, see Section 11.5, “Extensions for Spatial Data”.

12.15.1 Spatial Function Reference

Table 12.19 Spatial Functions

NameDescription
Area()Return Polygon or MultiPolygon area
AsBinary(), AsWKB()Convert from internal geometry format to WKB
AsText(), AsWKT()Convert from internal geometry format to WKT
Centroid()Return centroid as a point
Contains()Whether one geometry contains another
Crosses()Whether one geometry crosses another
Dimension()Dimension of geometry
Disjoint()Whether one geometry is disjoint from another
EndPoint()End Point of LineString
Envelope()Return MBR of geometry
Equals()Whether one geometry is equal to another
ExteriorRing()Return exterior ring of Polygon
GeomCollFromText(), GeometryCollectionFromText()Return geometry collection from WKT
GeomCollFromWKB(), GeometryCollectionFromWKB()Return geometry collection from WKB
GeometryCollection()Construct geometry collection from geometries
GeometryN()Return N-th geometry from geometry collection
GeometryType()Return name of geometry type
GeomFromText(), GeometryFromText()Return geometry from WKT
GeomFromWKB()Return geometry from WKB
GLength()Return length of LineString
InteriorRingN()Return N-th interior ring of Polygon
Intersects()Whether one geometry intersects another
IsClosed()Whether a geometry is closed and simple
IsEmpty()Placeholder function
IsSimple()Whether a geometry is simple
LineFromText()Construct LineString from WKT
LineFromWKB(), LineStringFromWKB()Construct LineString from WKB
LineString()Construct LineString from Point values
MBRContains()Whether MBR of one geometry contains MBR of another
MBRDisjoint()Whether MBRs of two geometries are disjoint
MBREqual() (deprecated 5.7.6)Whether MBRs of two geometries are equal
MBRIntersects()Whether MBRs of two geometries intersect
MBROverlaps()Whether MBRs of two geometries overlap
MBRTouches()Whether MBRs of two geometries touch
MBRWithin()Whether MBR of one geometry is within MBR of another
MLineFromText(), MultiLineStringFromText()Construct MultiLineString from WKT
MLineFromWKB(), MultiLineStringFromWKB()Construct MultiLineString from WKB
MPointFromText(), MultiPointFromText()Construct MultiPoint from WKT
MPointFromWKB(), MultiPointFromWKB()Construct MultiPoint from WKB
MPolyFromText(), MultiPolygonFromText()Construct MultiPolygon from WKT
MPolyFromWKB(), MultiPolygonFromWKB()Construct MultiPolygon from WKB
MultiLineString()Contruct MultiLineString from LineString values
MultiPoint()Construct MultiPoint from Point values
MultiPolygon()Construct MultiPolygon from Polygon values
NumGeometries()Return number of geometries in geometry collection
NumInteriorRings()Return number of interior rings in Polygon
NumPoints()Return number of points in LineString
Overlaps()Whether one geometry overlaps another
Point()Construct Point from coordinates
PointFromText()Construct Point from WKT
PointFromWKB()Construct Point from WKB
PointN()Return N-th point from LineString
PolyFromText(), PolygonFromText()Construct Polygon from WKT
PolyFromWKB(), PolygonFromWKB()Construct Polygon from WKB
Polygon()Construct Polygon from LineString arguments
SRID()Return spatial reference system ID for geometry
StartPoint()Start Point of LineString
Touches()Whether one geometry touches another
Within()Whether one geometry is within another
X()Return X coordinate of Point
Y()Return Y coordinate of Point

12.15.2 Argument Handling by Spatial Functions

Spatial values, or geometries, have the properties described at Section 11.5.2.2, “Geometry Class”. The following discussion lists general spatial function argument-handling characteristics. Specific functions or groups of functions may have additional argument-handling characteristics, as discussed in the sections where those function descriptions occur.

Spatial functions are defined only for valid geometry values. If an invalid geometry is passed to a spatial function, the result is undefined.

The Spatial Reference Identifier (SRID) of a geometry identifies the coordinate space in which the geometry is defined. In MySQL, the SRID value is an integer associated with the geometry value. However, all calculations are done assuming SRID 0, representing cartesian (planar) coordinates, regardless of the actual SRID value. In the future, calculations may use the specified SRID values. To ensure SRID 0 behavior, create geometries using SRID 0. SRID 0 is the default for new geometries if no SRID is specified.

The maximum usable SRID value is 232–1. If a larger value is given, only the lower 32 bits are used.

Geometry values produced by any spatial function inherit the SRID of the geometry arguments.

12.15.3 Functions That Create Geometry Values from WKT Values

These functions take as arguments a Well-Known Text (WKT) representation and, optionally, a spatial reference system identifier (SRID). They return the corresponding geometry.

GeomFromText() accepts a WKT value of any geometry type as its first argument. Other functions provide type-specific construction functions for construction of geometry values of each geometry type.

For a description of WKT format, see Section 11.5.3.1.1, “Well-Known Text (WKT) Format”.

12.15.4 Functions That Create Geometry Values from WKB Values

These functions take as arguments a BLOB containing a Well-Known Binary (WKB) representation and, optionally, a spatial reference system identifier (SRID). They return the corresponding geometry.

These functions also accept geometry objects for compatibility with the return value of the functions in Section 12.15.5, “MySQL-Specific Functions That Create Geometry Values”. Thus, those functions may be used to provide the first argument to the functions in this section.

GeomFromWKB() accepts a WKB value of any geometry type as its first argument. Other functions provide type-specific construction functions for construction of geometry values of each geometry type.

For a description of WKB format, see Section 11.5.3.1.2, “Well-Known Binary (WKB) Format”.

12.15.5 MySQL-Specific Functions That Create Geometry Values

MySQL provides a set of useful nonstandard functions for creating geometry values. The functions described in this section are MySQL extensions to the OpenGIS specification.

These functions produce geometry objects from either WKB values or geometry objects as arguments. If any argument is not a proper WKB or geometry representation of the proper object type, the return value is NULL.

For example, you can insert the geometry return value from Point() directly into a POINT column:

INSERT INTO t1 (pt_col) VALUES(Point(1,2));
  • GeometryCollection(g1,g2,...)

    Constructs a GeometryCollection.

    If the argument contains a nonsupported geometry, the return value is NULL.

  • LineString(pt1,pt2,...)

    Constructs a LineString value from a number of Point or WKB Point arguments. If the number of arguments is less than two, the return value is NULL.

  • MultiLineString(ls1,ls2,...)

    Constructs a MultiLineString value using LineString or WKB LineString arguments.

  • MultiPoint(pt1,pt2,...)

    Constructs a MultiPoint value using Point or WKB Point arguments.

  • MultiPolygon(poly1,poly2,...)

    Constructs a MultiPolygon value from a set of Polygon or WKB Polygon arguments.

  • Point(x,y)

    Constructs a Point using its coordinates.

  • Polygon(ls1,ls2,...)

    Constructs a Polygon value from a number of LineString or WKB LineString arguments. If any argument does not represent a LinearRing (that is, not a closed and simple LineString), the return value is NULL.

12.15.6 Geometry Format Conversion Functions

MySQL supports the following functions for converting geometry values between internal format and either WKT or WKB format:

12.15.7 Geometry Property Functions

Each function that belongs to this group takes a geometry value as its argument and returns some quantitative or qualitative property of the geometry. Some functions restrict their argument type. Such functions return NULL if the argument is of an incorrect geometry type. For example, the Area() polygon function returns NULL if the object type is neither Polygon nor MultiPolygon.

12.15.7.1 General Geometry Property Functions

The functions listed in this section do not restrict their argument and accept a geometry value of any type.

  • Dimension(g)

    Returns the inherent dimension of the geometry value g. The result can be –1, 0, 1, or 2. The meaning of these values is given in Section 11.5.2.2, “Geometry Class”.

    mysql> SELECT Dimension(GeomFromText('LineString(1 1,2 2)'));
    +------------------------------------------------+
    | Dimension(GeomFromText('LineString(1 1,2 2)')) |
    +------------------------------------------------+
    |                                              1 |
    +------------------------------------------------+
    
  • Envelope(g)

    Returns the minimum bounding rectangle (MBR) for the geometry value g. The result is returned as a Polygon value that is defined by the corner points of the bounding box:

    POLYGON((MINX MINY, MAXX MINY, MAXX MAXY, MINX MAXY, MINX MINY))
    
    mysql> SELECT AsText(Envelope(GeomFromText('LineString(1 1,2 2)')));
    +-------------------------------------------------------+
    | AsText(Envelope(GeomFromText('LineString(1 1,2 2)'))) |
    +-------------------------------------------------------+
    | POLYGON((1 1,2 1,2 2,1 2,1 1))                        |
    +-------------------------------------------------------+
    
  • GeometryType(g)

    Returns a binary string indicating the name of the geometry type of which the geometry instance g is a member. The name corresponds to one of the instantiable Geometry subclasses.

    mysql> SELECT GeometryType(GeomFromText('POINT(1 1)'));
    +------------------------------------------+
    | GeometryType(GeomFromText('POINT(1 1)')) |
    +------------------------------------------+
    | POINT                                    |
    +------------------------------------------+
    
  • IsEmpty(g)

    This function is a placeholder that returns 0 for any valid geometry value, 1 for any invalid geometry value or NULL.

    MySQL does not support GIS EMPTY values such as POINT EMPTY.

  • IsSimple(g)

    In MySQL 5.5, this function is a placeholder that always returns 0.

    The description of each instantiable geometric class given earlier in the chapter includes the specific conditions that cause an instance of that class to be classified as not simple. (See Section 11.5.2.1, “The Geometry Class Hierarchy”.)

  • SRID(g)

    Returns an integer indicating the Spatial Reference System ID for the geometry value g.

    In MySQL, the SRID value is just an integer associated with the geometry value. All calculations are done assuming Euclidean (planar) geometry.

    mysql> SELECT SRID(GeomFromText('LineString(1 1,2 2)',101));
    +-----------------------------------------------+
    | SRID(GeomFromText('LineString(1 1,2 2)',101)) |
    +-----------------------------------------------+
    |                                           101 |
    +-----------------------------------------------+
    

12.15.7.2 Point Property Functions

A Point consists of X and Y coordinates, which may be obtained using the following functions:

  • X(p)

    Returns the X-coordinate value for the Point object p as a double-precision number.

    mysql> SELECT X(POINT(56.7, 53.34));
    +-----------------------+
    | X(POINT(56.7, 53.34)) |
    +-----------------------+
    |                  56.7 |
    +-----------------------+
    
  • Y(p)

    Returns the Y-coordinate value for the Point object p as a double-precision number.

    mysql> SELECT Y(POINT(56.7, 53.34));
    +-----------------------+
    | Y(POINT(56.7, 53.34)) |
    +-----------------------+
    |                 53.34 |
    +-----------------------+
    

12.15.7.3 LineString Property Functions

A LineString consists of Point values. You can extract particular points of a LineString, count the number of points that it contains, or obtain its length.

  • EndPoint(ls)

    Returns the Point that is the endpoint of the LineString value ls.

    mysql> SET @ls = 'LineString(1 1,2 2,3 3)';
    mysql> SELECT AsText(EndPoint(GeomFromText(@ls)));
    +-------------------------------------+
    | AsText(EndPoint(GeomFromText(@ls))) |
    +-------------------------------------+
    | POINT(3 3)                          |
    +-------------------------------------+
    
  • GLength(ls)

    Returns a double-precision number indicating the length of the LineString value ls in its associated spatial reference.

    mysql> SET @ls = 'LineString(1 1,2 2,3 3)';
    mysql> SELECT GLength(GeomFromText(@ls));
    +----------------------------+
    | GLength(GeomFromText(@ls)) |
    +----------------------------+
    |            2.8284271247462 |
    +----------------------------+
    

    GLength() is a nonstandard name. It corresponds to the OpenGIS Length() function. (There is an existing SQL function Length() that calculates the length of string values.)

  • IsClosed(ls)

    Returns 1 if the LineString value ls is closed (that is, its StartPoint() and EndPoint() values are the same) and is simple (does not pass through the same point more than once). Returns 0 if ls is not closed, and –1 if it is NULL.

    mysql> SET @ls1 = 'LineString(1 1,2 2,3 3,2 2)';
    Query OK, 0 rows affected (0.00 sec)
    
    mysql> SET @ls2 = 'LineString(1 1,2 2,3 3,1 1)';
    Query OK, 0 rows affected (0.00 sec)
    
    mysql> SELECT IsClosed(GeomFromText(@ls1));
    +------------------------------+
    | IsClosed(GeomFromText(@ls1)) |
    +------------------------------+
    |                            0 |
    +------------------------------+
    1 row in set (0.00 sec)
    
    mysql> SELECT IsClosed(GeomFromText(@ls2));
    +------------------------------+
    | IsClosed(GeomFromText(@ls2)) |
    +------------------------------+
    |                            1 |
    +------------------------------+
    1 row in set (0.00 sec)
    
  • NumPoints(ls)

    Returns the number of Point objects in the LineString value ls.

    mysql> SET @ls = 'LineString(1 1,2 2,3 3)';
    mysql> SELECT NumPoints(GeomFromText(@ls));
    +------------------------------+
    | NumPoints(GeomFromText(@ls)) |
    +------------------------------+
    |                            3 |
    +------------------------------+
    
  • PointN(ls,N)

    Returns the N-th Point in the Linestring value ls. Points are numbered beginning with 1.

    mysql> SET @ls = 'LineString(1 1,2 2,3 3)';
    mysql> SELECT AsText(PointN(GeomFromText(@ls),2));
    +-------------------------------------+
    | AsText(PointN(GeomFromText(@ls),2)) |
    +-------------------------------------+
    | POINT(2 2)                          |
    +-------------------------------------+
    
  • StartPoint(ls)

    Returns the Point that is the start point of the LineString value ls.

    mysql> SET @ls = 'LineString(1 1,2 2,3 3)';
    mysql> SELECT AsText(StartPoint(GeomFromText(@ls)));
    +---------------------------------------+
    | AsText(StartPoint(GeomFromText(@ls))) |
    +---------------------------------------+
    | POINT(1 1)                            |
    +---------------------------------------+
    

12.15.7.4 MultiLineString Property Functions

These functions return properties of MultiLineString values.

  • GLength(mls)

    Returns a double-precision number indicating the length of the MultiLineString value mls. The length of mls is equal to the sum of the lengths of its elements.

    mysql> SET @mls = 'MultiLineString((1 1,2 2,3 3),(4 4,5 5))';
    mysql> SELECT GLength(GeomFromText(@mls));
    +-----------------------------+
    | GLength(GeomFromText(@mls)) |
    +-----------------------------+
    |             4.2426406871193 |
    +-----------------------------+
    

    GLength() is a nonstandard name. It corresponds to the OpenGIS Length() function.

  • IsClosed(mls)

    Returns 1 if the MultiLineString value mls is closed (that is, the StartPoint() and EndPoint() values are the same for each LineString in mls). Returns 0 if mls is not closed, and –1 if it is NULL.

    mysql> SET @mls = 'MultiLineString((1 1,2 2,3 3),(4 4,5 5))';
    mysql> SELECT IsClosed(GeomFromText(@mls));
    +------------------------------+
    | IsClosed(GeomFromText(@mls)) |
    +------------------------------+
    |                            0 |
    +------------------------------+
    

12.15.7.5 Polygon Property Functions

These functions return properties of Polygon values.

  • Area(poly)

    Returns a double-precision number indicating the area of the argument, as measured in its spatial reference system. For arguments of dimension 0 or 1, the result is 0.

    mysql> SET @poly = 'Polygon((0 0,0 3,3 0,0 0),(1 1,1 2,2 1,1 1))';
    mysql> SELECT Area(GeomFromText(@poly));
    +---------------------------+
    | Area(GeomFromText(@poly)) |
    +---------------------------+
    |                         4 |
    +---------------------------+
    
    mysql> SET @mpoly =
        -> 'MultiPolygon(((0 0,0 3,3 3,3 0,0 0),(1 1,1 2,2 2,2 1,1 1)))';
    mysql> SELECT Area(GeomFromText(@mpoly));
    +----------------------------+
    | Area(GeomFromText(@mpoly)) |
    +----------------------------+
    |                          8 |
    +----------------------------+
    
  • ExteriorRing(poly)

    Returns the exterior ring of the Polygon value poly as a LineString.

    mysql> SET @poly =
        -> 'Polygon((0 0,0 3,3 3,3 0,0 0),(1 1,1 2,2 2,2 1,1 1))';
    mysql> SELECT AsText(ExteriorRing(GeomFromText(@poly)));
    +-------------------------------------------+
    | AsText(ExteriorRing(GeomFromText(@poly))) |
    +-------------------------------------------+
    | LINESTRING(0 0,0 3,3 3,3 0,0 0)           |
    +-------------------------------------------+
    
  • InteriorRingN(poly,N)

    Returns the N-th interior ring for the Polygon value poly as a LineString. Rings are numbered beginning with 1.

    mysql> SET @poly =
        -> 'Polygon((0 0,0 3,3 3,3 0,0 0),(1 1,1 2,2 2,2 1,1 1))';
    mysql> SELECT AsText(InteriorRingN(GeomFromText(@poly),1));
    +----------------------------------------------+
    | AsText(InteriorRingN(GeomFromText(@poly),1)) |
    +----------------------------------------------+
    | LINESTRING(1 1,1 2,2 2,2 1,1 1)              |
    +----------------------------------------------+
    
  • NumInteriorRings(poly)

    Returns the number of interior rings in the Polygon value poly.

    mysql> SET @poly =
        -> 'Polygon((0 0,0 3,3 3,3 0,0 0),(1 1,1 2,2 2,2 1,1 1))';
    mysql> SELECT NumInteriorRings(GeomFromText(@poly));
    +---------------------------------------+
    | NumInteriorRings(GeomFromText(@poly)) |
    +---------------------------------------+
    |                                     1 |
    +---------------------------------------+
    

12.15.7.6 MultiPolygon Property Functions

These functions return properties of MultiPolygon values.

  • Area(mpoly)

    See the description of Area() in Section 12.15.7.5, “Polygon Property Functions”.

  • Centroid(mpoly)

    Returns the mathematical centroid for the MultiPolygon value mpoly as a Point. The result is not guaranteed to be on the MultiPolygon.

    mysql> SET @poly =
        -> GeomFromText('POLYGON((0 0,10 0,10 10,0 10,0 0),(5 5,7 5,7 7,5 7,5 5))');
    mysql> SELECT GeometryType(@poly),AsText(Centroid(@poly));
    +---------------------+--------------------------------------------+
    | GeometryType(@poly) | AsText(Centroid(@poly))                    |
    +---------------------+--------------------------------------------+
    | POLYGON             | POINT(4.958333333333333 4.958333333333333) |
    +---------------------+--------------------------------------------+
    

12.15.7.7 GeometryCollection Property Functions

These functions return properties of GeometryCollection values.

  • GeometryN(gc,N)

    Returns the N-th geometry in the GeometryCollection value gc. Geometries are numbered beginning with 1.

    mysql> SET @gc = 'GeometryCollection(Point(1 1),LineString(2 2, 3 3))';
    mysql> SELECT AsText(GeometryN(GeomFromText(@gc),1));
    +----------------------------------------+
    | AsText(GeometryN(GeomFromText(@gc),1)) |
    +----------------------------------------+
    | POINT(1 1)                             |
    +----------------------------------------+
    
  • NumGeometries(gc)

    Returns the number of geometries in the GeometryCollection value gc.

    mysql> SET @gc = 'GeometryCollection(Point(1 1),LineString(2 2, 3 3))';
    mysql> SELECT NumGeometries(GeomFromText(@gc));
    +----------------------------------+
    | NumGeometries(GeomFromText(@gc)) |
    +----------------------------------+
    |                                2 |
    +----------------------------------+
    

12.15.8 Spatial Operator Functions

Section 12.15.7, “Geometry Property Functions”, discusses several functions that construct new geometries from existing ones. See that section for descriptions of these functions:

12.15.9 Functions That Test Spatial Relations Between Geometry Objects

The functions described in this section take two geometries as arguments and return a qualitative or quantitative relation between them.

  • The OpenGIS specification defines the a set of functions that test the relationship between two geometry values. MySQL originally implemented these functions such that they used object minimum bounding rectangles (MBRs) and returned the same result as the corresponding MBR-based functions. For example, Contains() and MBRContains() return the same result. (Exception: Crosses() exists but MBRCrosses() does not.)

  • A MySQL-specific set of MBR-based functions is available to test the relationship between two geometry values.

12.15.9.1 Spatial Relation Functions That Use Minimum Bounding Rectangles (MBRs)

The OpenGIS specification defines the following functions. They test the relationship between two geometry values g1 and g2. The MySQL implementation uses minimum bounding rectangles, so these functions return the same result as the corresponding MBR-based functions. The return values 1 and 0 indicate true and false, respectively.

  • Contains(g1,g2)

    Returns 1 or 0 to indicate whether g1 completely contains g2. This tests the opposite relationship as Within().

  • Crosses(g1,g2)

    Returns 1 if g1 spatially crosses g2. Returns NULL if g1 is a Polygon or a MultiPolygon, or if g2 is a Point or a MultiPoint. Otherwise, returns 0.

    The term spatially crosses denotes a spatial relation between two given geometries that has the following properties:

    • The two geometries intersect

    • Their intersection results in a geometry that has a dimension that is one less than the maximum dimension of the two given geometries

    • Their intersection is not equal to either of the two given geometries

  • Disjoint(g1,g2)

    Returns 1 or 0 to indicate whether g1 is spatially disjoint from (does not intersect) g2.

  • Equals(g1,g2)

    Returns 1 or 0 to indicate whether g1 is spatially equal to g2.

  • Intersects(g1,g2)

    Returns 1 or 0 to indicate whether g1 spatially intersects g2.

  • Overlaps(g1,g2)

    Returns 1 or 0 to indicate whether g1 spatially overlaps g2. The term spatially overlaps is used if two geometries intersect and their intersection results in a geometry of the same dimension but not equal to either of the given geometries.

  • Touches(g1,g2)

    Returns 1 or 0 to indicate whether g1 spatially touches g2. Two geometries spatially touch if the interiors of the geometries do not intersect, but the boundary of one of the geometries intersects either the boundary or the interior of the other.

  • Within(g1,g2)

    Returns 1 or 0 to indicate whether g1 is spatially within g2. This tests the opposite relationship as Contains().

12.15.9.2 MySQL-Specific Spatial Relation Functions That Use Minimum Bounding Rectangles (MBRs)

MySQL provides several functions that test relations between minimum bounding rectangles of two geometries g1 and g2. The return values 1 and 0 indicate true and false, respectively.

  • MBRContains(g1,g2)

    Returns 1 or 0 to indicate whether the minimum bounding rectangle of g1 contains the minimum bounding rectangle of g2. This tests the opposite relationship as MBRWithin().

    mysql> SET @g1 = GeomFromText('Polygon((0 0,0 3,3 3,3 0,0 0))');
    mysql> SET @g2 = GeomFromText('Point(1 1)');
    mysql> SELECT MBRContains(@g1,@g2), MBRWithin(@g2,@g1);
    +----------------------+--------------------+
    | MBRContains(@g1,@g2) | MBRWithin(@g2,@g1) |
    +----------------------+--------------------+
    |                    1 |                  1 |
    +----------------------+--------------------+
    
  • MBRDisjoint(g1,g2)

    Returns 1 or 0 to indicate whether the minimum bounding rectangles of the two geometries g1 and g2 are disjoint (do not intersect).

  • MBREqual(g1,g2)

    Returns 1 or 0 to indicate whether the minimum bounding rectangles of the two geometries g1 and g2 are the same.

  • MBRIntersects(g1,g2)

    Returns 1 or 0 to indicate whether the minimum bounding rectangles of the two geometries g1 and g2 intersect.

  • MBROverlaps(g1,g2)

    Returns 1 or 0 to indicate whether the minimum bounding rectangles of the two geometries g1 and g2 overlap. The term spatially overlaps is used if two geometries intersect and their intersection results in a geometry of the same dimension but not equal to either of the given geometries.

  • MBRTouches(g1,g2)

    Returns 1 or 0 to indicate whether the minimum bounding rectangles of the two geometries g1 and g2 touch. Two geometries spatially touch if the interiors of the geometries do not intersect, but the boundary of one of the geometries intersects either the boundary or the interior of the other.

  • MBRWithin(g1,g2)

    Returns 1 or 0 to indicate whether the minimum bounding rectangle of g1 is within the minimum bounding rectangle of g2. This tests the opposite relationship as MBRContains().

    mysql> SET @g1 = GeomFromText('Polygon((0 0,0 3,3 3,3 0,0 0))');
    mysql> SET @g2 = GeomFromText('Polygon((0 0,0 5,5 5,5 0,0 0))');
    mysql> SELECT MBRWithin(@g1,@g2), MBRWithin(@g2,@g1);
    +--------------------+--------------------+
    | MBRWithin(@g1,@g2) | MBRWithin(@g2,@g1) |
    +--------------------+--------------------+
    |                  1 |                  0 |
    +--------------------+--------------------+
    

12.16 Miscellaneous Functions

Table 12.20 Miscellaneous Functions

NameDescription
DEFAULT()Return the default value for a table column
GET_LOCK()Get a named lock
INET_ATON()Return the numeric value of an IP address
INET_NTOA()Return the IP address from a numeric value
IS_FREE_LOCK()Checks whether the named lock is free
IS_USED_LOCK()Checks whether the named lock is in use. Return connection identifier if true.
MASTER_POS_WAIT()Block until the slave has read and applied all updates up to the specified position
NAME_CONST()Causes the column to have the given name
RAND()Return a random floating-point value
RELEASE_LOCK()Releases the named lock
SLEEP()Sleep for a number of seconds
UUID_SHORT()Return an integer-valued universal identifier
UUID()Return a Universal Unique Identifier (UUID)
VALUES()Defines the values to be used during an INSERT

  • DEFAULT(col_name)

    Returns the default value for a table column. An error results if the column has no default value.

    mysql> UPDATE t SET i = DEFAULT(i)+1 WHERE id < 100;
    
  • FORMAT(X,D)

    Formats the number X to a format like '#,###,###.##', rounded to D decimal places, and returns the result as a string. For details, see Section 12.5, “String Functions”.

  • GET_LOCK(str,timeout)

    Tries to obtain a lock with a name given by the string str, using a timeout of timeout seconds. Before MySQL 5.5.8, a negative timeout value means infinite timeout on Windows. As of 5.5.8, this behavior applies on all platforms.

    Returns 1 if the lock was obtained successfully, 0 if the attempt timed out (for example, because another client has previously locked the name), or NULL if an error occurred (such as running out of memory or the thread was killed with mysqladmin kill). If you have a lock obtained with GET_LOCK(), it is released when you execute RELEASE_LOCK(), execute a new GET_LOCK(), or your connection terminates (either normally or abnormally). Locks obtained with GET_LOCK() do not interact with transactions. That is, committing a transaction does not release any such locks obtained during the transaction.

    GET_LOCK() can be used to implement application locks or to simulate record locks. Names are locked on a server-wide basis. If a name has been locked within one session, GET_LOCK() blocks any request by another session for a lock with the same name. This enables clients that agree on a given lock name to use the name to perform cooperative advisory locking. But be aware that it also enables a client that is not among the set of cooperating clients to lock a name, either inadvertently or deliberately, and thus prevent any of the cooperating clients from locking that name. One way to reduce the likelihood of this is to use lock names that are database-specific or application-specific. For example, use lock names of the form db_name.str or app_name.str.

    mysql> SELECT GET_LOCK('lock1',10);
            -> 1
    mysql> SELECT IS_FREE_LOCK('lock2');
            -> 1
    mysql> SELECT GET_LOCK('lock2',10);
            -> 1
    mysql> SELECT RELEASE_LOCK('lock2');
            -> 1
    mysql> SELECT RELEASE_LOCK('lock1');
            -> NULL
    

    The second RELEASE_LOCK() call returns NULL because the lock 'lock1' was automatically released by the second GET_LOCK() call.

    If multiple clients are waiting for a lock, the order in which they will acquire it is undefined. Applications should not assume that clients will acquire the lock in the same order that they issued the lock requests.

    Note

    Before MySQL 5.5.3, if a client attempts to acquire a lock that is already held by another client, it blocks according to the timeout argument. If the blocked client terminates, its thread does not die until the lock request times out.

    This function is unsafe for statement-based replication. Beginning with MySQL 5.5.1, a warning is logged if you use this function when binlog_format is set to STATEMENT. (Bug #47995)

  • INET_ATON(expr)

    Given the dotted-quad representation of an IPv4 network address as a string, returns an integer that represents the numeric value of the address in network byte order (big endian). INET_ATON() returns NULL if it does not understand its argument.

    mysql> SELECT INET_ATON('10.0.5.9');
            -> 167773449
    

    For this example, the return value is calculated as 10×2563 + 0×2562 + 5×256 + 9.

    INET_ATON() may or may not return a non-NULL result for short-form IP addresses (such as '127.1' as a representation of '127.0.0.1'). Because of this, INET_ATON()a should not be used for such addresses.

    Note

    To store values generated by INET_ATON(), use an INT UNSIGNED column rather than INT, which is signed. If you use a signed column, values corresponding to IP addresses for which the first octet is greater than 127 cannot be stored correctly. See Section 11.2.6, “Out-of-Range and Overflow Handling”.

  • INET_NTOA(expr)

    Given a numeric IPv4 network address in network byte order, returns the dotted-quad representation of the address as a string. INET_NTOA() returns NULL if it does not understand its argument.

    As of MySQL 5.5.3, the return value is a nonbinary string in the connection character set. Before 5.5.3, the return value is a binary string.

    mysql> SELECT INET_NTOA(167773449);
            -> '10.0.5.9'
    
  • IS_FREE_LOCK(str)

    Checks whether the lock named str is free to use (that is, not locked). Returns 1 if the lock is free (no one is using the lock), 0 if the lock is in use, and NULL if an error occurs (such as an incorrect argument).

    This function is unsafe for statement-based replication. Beginning with MySQL 5.5.1, a warning is logged if you use this function when binlog_format is set to STATEMENT. (Bug #47995)

  • IS_USED_LOCK(str)

    Checks whether the lock named str is in use (that is, locked). If so, it returns the connection identifier of the client that holds the lock. Otherwise, it returns NULL.

    This function is unsafe for statement-based replication. Beginning with MySQL 5.5.1, a warning is logged if you use this function when binlog_format is set to STATEMENT. (Bug #47995)

  • MASTER_POS_WAIT(log_name,log_pos[,timeout])

    This function is useful for control of master/slave synchronization. It blocks until the slave has read and applied all updates up to the specified position in the master log. The return value is the number of log events the slave had to wait for to advance to the specified position. The function returns NULL if the slave SQL thread is not started, the slave's master information is not initialized, the arguments are incorrect, or an error occurs. It returns -1 if the timeout has been exceeded. If the slave SQL thread stops while MASTER_POS_WAIT() is waiting, the function returns NULL. If the slave is past the specified position, the function returns immediately.

    If a timeout value is specified, MASTER_POS_WAIT() stops waiting when timeout seconds have elapsed. timeout must be greater than 0; a zero or negative timeout means no timeout.

    This function is unsafe for statement-based replication. Beginning with MySQL 5.5.1, a warning is logged if you use this function when binlog_format is set to STATEMENT. (Bug #47995)

  • NAME_CONST(name,value)

    Returns the given value. When used to produce a result set column, NAME_CONST() causes the column to have the given name. The arguments should be constants.

    mysql> SELECT NAME_CONST('myname', 14);
    +--------+
    | myname |
    +--------+
    |     14 |
    +--------+
    

    This function is for internal use only. The server uses it when writing statements from stored programs that contain references to local program variables, as described in Section 20.7, “Binary Logging of Stored Programs”, You might see this function in the output from mysqlbinlog.

    For your applications, you can obtain exactly the same result as in the example just shown by using simple aliasing, like this:

    mysql> SELECT 14 AS myname;
    +--------+
    | myname |
    +--------+
    |     14 |
    +--------+
    1 row in set (0.00 sec)
    

    See Section 13.2.9, “SELECT Syntax”, for more information about column aliases.

  • RELEASE_LOCK(str)

    Releases the lock named by the string str that was obtained with GET_LOCK(). Returns 1 if the lock was released, 0 if the lock was not established by this thread (in which case the lock is not released), and NULL if the named lock did not exist. The lock does not exist if it was never obtained by a call to GET_LOCK() or if it has previously been released.

    The DO statement is convenient to use with RELEASE_LOCK(). See Section 13.2.3, “DO Syntax”.

    This function is unsafe for statement-based replication. Beginning with MySQL 5.5.1, a warning is logged if you use this function when binlog_format is set to STATEMENT. (Bug #47995)

  • SLEEP(duration)

    Sleeps (pauses) for the number of seconds given by the duration argument, then returns 0. If SLEEP() is interrupted, it returns 1. The duration may have a fractional part.

    This function is unsafe for statement-based replication. Beginning with MySQL 5.5.1, a warning is logged if you use this function when binlog_format is set to STATEMENT. (Bug #47995)

  • UUID()

    Returns a Universal Unique Identifier (UUID) generated according to DCE 1.1: Remote Procedure Call (Appendix A) CAE (Common Applications Environment) Specifications published by The Open Group in October 1997 (Document Number C706, http://www.opengroup.org/public/pubs/catalog/c706.htm).

    A UUID is designed as a number that is globally unique in space and time. Two calls to UUID() are expected to generate two different values, even if these calls are performed on two separate computers that are not connected to each other.

    A UUID is a 128-bit number represented by a utf8 string of five hexadecimal numbers in aaaaaaaa-bbbb-cccc-dddd-eeeeeeeeeeee format:

    • The first three numbers are generated from a timestamp.

    • The fourth number preserves temporal uniqueness in case the timestamp value loses monotonicity (for example, due to daylight saving time).

    • The fifth number is an IEEE 802 node number that provides spatial uniqueness. A random number is substituted if the latter is not available (for example, because the host computer has no Ethernet card, or we do not know how to find the hardware address of an interface on your operating system). In this case, spatial uniqueness cannot be guaranteed. Nevertheless, a collision should have very low probability.

      Currently, the MAC address of an interface is taken into account only on FreeBSD and Linux. On other operating systems, MySQL uses a randomly generated 48-bit number.

    mysql> SELECT UUID();
            -> '6ccd780c-baba-1026-9564-0040f4311e29'
    
    Warning

    Although UUID() values are intended to be unique, they are not necessarily unguessable or unpredictable. If unpredictability is required, UUID values should be generated some other way.

    Note

    UUID() does not work with statement-based replication.

  • UUID_SHORT()

    Returns a short universal identifier as a 64-bit unsigned integer (rather than a string-form 128-bit identifier as returned by the UUID() function).

    The value of UUID_SHORT() is guaranteed to be unique if the following conditions hold:

    • The server_id of the current host is unique among your set of master and slave servers

    • server_id is between 0 and 255

    • You do not set back your system time for your server between mysqld restarts

    • You do not invoke UUID_SHORT() on average more than 16 million times per second between mysqld restarts

    The UUID_SHORT() return value is constructed this way:

      (server_id & 255) << 56
    + (server_startup_time_in_seconds << 24)
    + incremented_variable++;
    
    mysql> SELECT UUID_SHORT();
            -> 92395783831158784
    

    Note that UUID_SHORT() does not work with statement-based replication.

  • VALUES(col_name)

    In an INSERT ... ON DUPLICATE KEY UPDATE statement, you can use the VALUES(col_name) function in the UPDATE clause to refer to column values from the INSERT portion of the statement. In other words, VALUES(col_name) in the UPDATE clause refers to the value of col_name that would be inserted, had no duplicate-key conflict occurred. This function is especially useful in multiple-row inserts. The VALUES() function is meaningful only in the ON DUPLICATE KEY UPDATE clause of INSERT statements and returns NULL otherwise. See Section 13.2.5.3, “INSERT ... ON DUPLICATE KEY UPDATE Syntax”.

    mysql> INSERT INTO table (a,b,c) VALUES (1,2,3),(4,5,6)
        -> ON DUPLICATE KEY UPDATE c=VALUES(a)+VALUES(b);
    

12.17 Functions and Modifiers for Use with GROUP BY Clauses

12.17.1 GROUP BY (Aggregate) Functions

Table 12.21 Aggregate (GROUP BY) Functions

NameDescription
AVG()Return the average value of the argument
BIT_AND()Return bitwise and
BIT_OR()Return bitwise or
BIT_XOR()Return bitwise xor
COUNT(DISTINCT)Return the count of a number of different values
COUNT()Return a count of the number of rows returned
GROUP_CONCAT()Return a concatenated string
MAX()Return the maximum value
MIN()Return the minimum value
STD()Return the population standard deviation
STDDEV_POP()Return the population standard deviation
STDDEV_SAMP()Return the sample standard deviation
STDDEV()Return the population standard deviation
SUM()Return the sum
VAR_POP()Return the population standard variance
VAR_SAMP()Return the sample variance
VARIANCE()Return the population standard variance

This section describes group (aggregate) functions that operate on sets of values. Unless otherwise stated, group functions ignore NULL values.

If you use a group function in a statement containing no GROUP BY clause, it is equivalent to grouping on all rows. For more information, see Section 12.17.3, “MySQL Handling of GROUP BY”.

For numeric arguments, the variance and standard deviation functions return a DOUBLE value. The SUM() and AVG() functions return a DECIMAL value for exact-value arguments (integer or DECIMAL), and a DOUBLE value for approximate-value arguments (FLOAT or DOUBLE).

The SUM() and AVG() aggregate functions do not work with temporal values. (They convert the values to numbers, losing everything after the first nonnumeric character.) To work around this problem, convert to numeric units, perform the aggregate operation, and convert back to a temporal value. Examples:

SELECT SEC_TO_TIME(SUM(TIME_TO_SEC(time_col))) FROM tbl_name;
SELECT FROM_DAYS(SUM(TO_DAYS(date_col))) FROM tbl_name;

Functions such as SUM() or AVG() that expect a numeric argument cast the argument to a number if necessary. For SET or ENUM values, the cast operation causes the underlying numeric value to be used.

  • AVG([DISTINCT] expr)

    Returns the average value of expr. The DISTINCT option can be used to return the average of the distinct values of expr.

    AVG() returns NULL if there were no matching rows.

    mysql> SELECT student_name, AVG(test_score)
        ->        FROM student
        ->        GROUP BY student_name;
    
  • BIT_AND(expr)

    Returns the bitwise AND of all bits in expr. The calculation is performed with 64-bit (BIGINT) precision.

    This function returns 18446744073709551615 if there were no matching rows. (This is the value of an unsigned BIGINT value with all bits set to 1.)

  • BIT_OR(expr)

    Returns the bitwise OR of all bits in expr. The calculation is performed with 64-bit (BIGINT) precision.

    This function returns 0 if there were no matching rows.

  • BIT_XOR(expr)

    Returns the bitwise XOR of all bits in expr. The calculation is performed with 64-bit (BIGINT) precision.

    This function returns 0 if there were no matching rows.

  • COUNT(expr)

    Returns a count of the number of non-NULL values of expr in the rows retrieved by a SELECT statement. The result is a BIGINT value.

    COUNT() returns 0 if there were no matching rows.

    mysql> SELECT student.student_name,COUNT(*)
        ->        FROM student,course
        ->        WHERE student.student_id=course.student_id
        ->        GROUP BY student_name;
    

    COUNT(*) is somewhat different in that it returns a count of the number of rows retrieved, whether or not they contain NULL values.

    COUNT(*) is optimized to return very quickly if the SELECT retrieves from one table, no other columns are retrieved, and there is no WHERE clause. For example:

    mysql> SELECT COUNT(*) FROM student;
    

    This optimization applies only to MyISAM tables only, because an exact row count is stored for this storage engine and can be accessed very quickly. For transactional storage engines such as InnoDB, storing an exact row count is more problematic because multiple transactions may be occurring, each of which may affect the count.

  • COUNT(DISTINCT expr,[expr...])

    Returns a count of the number of rows with different non-NULL expr values.

    COUNT(DISTINCT) returns 0 if there were no matching rows.

    mysql> SELECT COUNT(DISTINCT results) FROM student;
    

    In MySQL, you can obtain the number of distinct expression combinations that do not contain NULL by giving a list of expressions. In standard SQL, you would have to do a concatenation of all expressions inside COUNT(DISTINCT ...).

  • GROUP_CONCAT(expr)

    This function returns a string result with the concatenated non-NULL values from a group. It returns NULL if there are no non-NULL values. The full syntax is as follows:

    GROUP_CONCAT([DISTINCT] expr [,expr ...]
                 [ORDER BY {unsigned_integer | col_name | expr}
                     [ASC | DESC] [,col_name ...]]
                 [SEPARATOR str_val])
    
    mysql> SELECT student_name,
        ->     GROUP_CONCAT(test_score)
        ->     FROM student
        ->     GROUP BY student_name;
    

    Or:

    mysql> SELECT student_name,
        ->     GROUP_CONCAT(DISTINCT test_score
        ->               ORDER BY test_score DESC SEPARATOR ' ')
        ->     FROM student
        ->     GROUP BY student_name;
    

    In MySQL, you can get the concatenated values of expression combinations. To eliminate duplicate values, use the DISTINCT clause. To sort values in the result, use the ORDER BY clause. To sort in reverse order, add the DESC (descending) keyword to the name of the column you are sorting by in the ORDER BY clause. The default is ascending order; this may be specified explicitly using the ASC keyword. The default separator between values in a group is comma (,). To specify a separator explicitly, use SEPARATOR followed by the string literal value that should be inserted between group values. To eliminate the separator altogether, specify SEPARATOR ''.

    The result is truncated to the maximum length that is given by the group_concat_max_len system variable, which has a default value of 1024. The value can be set higher, although the effective maximum length of the return value is constrained by the value of max_allowed_packet. The syntax to change the value of group_concat_max_len at runtime is as follows, where val is an unsigned integer:

    SET [GLOBAL | SESSION] group_concat_max_len = val;
    

    The return value is a nonbinary or binary string, depending on whether the arguments are nonbinary or binary strings. The result type is TEXT or BLOB unless group_concat_max_len is less than or equal to 512, in which case the result type is VARCHAR or VARBINARY.

    See also CONCAT() and CONCAT_WS(): Section 12.5, “String Functions”.

  • MAX([DISTINCT] expr)

    Returns the maximum value of expr. MAX() may take a string argument; in such cases, it returns the maximum string value. See Section 8.3.1, “How MySQL Uses Indexes”. The DISTINCT keyword can be used to find the maximum of the distinct values of expr, however, this produces the same result as omitting DISTINCT.

    MAX() returns NULL if there were no matching rows.

    mysql> SELECT student_name, MIN(test_score), MAX(test_score)
        ->        FROM student
        ->        GROUP BY student_name;
    

    For MAX(), MySQL currently compares ENUM and SET columns by their string value rather than by the string's relative position in the set. This differs from how ORDER BY compares them. This is expected to be rectified in a future MySQL release.

  • MIN([DISTINCT] expr)

    Returns the minimum value of expr. MIN() may take a string argument; in such cases, it returns the minimum string value. See Section 8.3.1, “How MySQL Uses Indexes”. The DISTINCT keyword can be used to find the minimum of the distinct values of expr, however, this produces the same result as omitting DISTINCT.

    MIN() returns NULL if there were no matching rows.

    mysql> SELECT student_name, MIN(test_score), MAX(test_score)
        ->        FROM student
        ->        GROUP BY student_name;
    

    For MIN(), MySQL currently compares ENUM and SET columns by their string value rather than by the string's relative position in the set. This differs from how ORDER BY compares them. This is expected to be rectified in a future MySQL release.

  • STD(expr)

    Returns the population standard deviation of expr. This is an extension to standard SQL. The standard SQL function STDDEV_POP() can be used instead.

    This function returns NULL if there were no matching rows.

  • STDDEV(expr)

    Returns the population standard deviation of expr. This function is provided for compatibility with Oracle. The standard SQL function STDDEV_POP() can be used instead.

    This function returns NULL if there were no matching rows.

  • STDDEV_POP(expr)

    Returns the population standard deviation of expr (the square root of VAR_POP()). You can also use STD() or STDDEV(), which are equivalent but not standard SQL.

    STDDEV_POP() returns NULL if there were no matching rows.

  • STDDEV_SAMP(expr)

    Returns the sample standard deviation of expr (the square root of VAR_SAMP().

    STDDEV_SAMP() returns NULL if there were no matching rows.

  • SUM([DISTINCT] expr)

    Returns the sum of expr. If the return set has no rows, SUM() returns NULL. The DISTINCT keyword can be used to sum only the distinct values of expr.

    SUM() returns NULL if there were no matching rows.

  • VAR_POP(expr)

    Returns the population standard variance of expr. It considers rows as the whole population, not as a sample, so it has the number of rows as the denominator. You can also use VARIANCE(), which is equivalent but is not standard SQL.

    VAR_POP() returns NULL if there were no matching rows.

  • VAR_SAMP(expr)

    Returns the sample variance of expr. That is, the denominator is the number of rows minus one.

    VAR_SAMP() returns NULL if there were no matching rows.

  • VARIANCE(expr)

    Returns the population standard variance of expr. This is an extension to standard SQL. The standard SQL function VAR_POP() can be used instead.

    VARIANCE() returns NULL if there were no matching rows.

12.17.2 GROUP BY Modifiers

The GROUP BY clause permits a WITH ROLLUP modifier that causes extra rows to be added to the summary output. These rows represent higher-level (or super-aggregate) summary operations. ROLLUP thus enables you to answer questions at multiple levels of analysis with a single query. It can be used, for example, to provide support for OLAP (Online Analytical Processing) operations.

Suppose that a table named sales has year, country, product, and profit columns for recording sales profitability:

CREATE TABLE sales
(
    year    INT NOT NULL,
    country VARCHAR(20) NOT NULL,
    product VARCHAR(32) NOT NULL,
    profit  INT
);

The table's contents can be summarized per year with a simple GROUP BY like this:

mysql> SELECT year, SUM(profit) FROM sales GROUP BY year;
+------+-------------+
| year | SUM(profit) |
+------+-------------+
| 2000 |        4525 |
| 2001 |        3010 |
+------+-------------+

This output shows the total profit for each year, but if you also want to determine the total profit summed over all years, you must add up the individual values yourself or run an additional query.

Or you can use ROLLUP, which provides both levels of analysis with a single query. Adding a WITH ROLLUP modifier to the GROUP BY clause causes the query to produce another row that shows the grand total over all year values:

mysql> SELECT year, SUM(profit) FROM sales GROUP BY year WITH ROLLUP;
+------+-------------+
| year | SUM(profit) |
+------+-------------+
| 2000 |        4525 |
| 2001 |        3010 |
| NULL |        7535 |
+------+-------------+

The grand total super-aggregate line is identified by the value NULL in the year column.

ROLLUP has a more complex effect when there are multiple GROUP BY columns. In this case, each time there is a break (change in value) in any but the last grouping column, the query produces an extra super-aggregate summary row.

For example, without ROLLUP, a summary on the sales table based on year, country, and product might look like this:

mysql> SELECT year, country, product, SUM(profit)
    -> FROM sales
    -> GROUP BY year, country, product;
+------+---------+------------+-------------+
| year | country | product    | SUM(profit) |
+------+---------+------------+-------------+
| 2000 | Finland | Computer   |        1500 |
| 2000 | Finland | Phone      |         100 |
| 2000 | India   | Calculator |         150 |
| 2000 | India   | Computer   |        1200 |
| 2000 | USA     | Calculator |          75 |
| 2000 | USA     | Computer   |        1500 |
| 2001 | Finland | Phone      |          10 |
| 2001 | USA     | Calculator |          50 |
| 2001 | USA     | Computer   |        2700 |
| 2001 | USA     | TV         |         250 |
+------+---------+------------+-------------+

The output indicates summary values only at the year/country/product level of analysis. When ROLLUP is added, the query produces several extra rows:

mysql> SELECT year, country, product, SUM(profit)
    -> FROM sales
    -> GROUP BY year, country, product WITH ROLLUP;
+------+---------+------------+-------------+
| year | country | product    | SUM(profit) |
+------+---------+------------+-------------+
| 2000 | Finland | Computer   |        1500 |
| 2000 | Finland | Phone      |         100 |
| 2000 | Finland | NULL       |        1600 |
| 2000 | India   | Calculator |         150 |
| 2000 | India   | Computer   |        1200 |
| 2000 | India   | NULL       |        1350 |
| 2000 | USA     | Calculator |          75 |
| 2000 | USA     | Computer   |        1500 |
| 2000 | USA     | NULL       |        1575 |
| 2000 | NULL    | NULL       |        4525 |
| 2001 | Finland | Phone      |          10 |
| 2001 | Finland | NULL       |          10 |
| 2001 | USA     | Calculator |          50 |
| 2001 | USA     | Computer   |        2700 |
| 2001 | USA     | TV         |         250 |
| 2001 | USA     | NULL       |        3000 |
| 2001 | NULL    | NULL       |        3010 |
| NULL | NULL    | NULL       |        7535 |
+------+---------+------------+-------------+

For this query, adding ROLLUP causes the output to include summary information at four levels of analysis, not just one. Here is how to interpret the ROLLUP output:

  • Following each set of product rows for a given year and country, an extra summary row is produced showing the total for all products. These rows have the product column set to NULL.

  • Following each set of rows for a given year, an extra summary row is produced showing the total for all countries and products. These rows have the country and products columns set to NULL.

  • Finally, following all other rows, an extra summary row is produced showing the grand total for all years, countries, and products. This row has the year, country, and products columns set to NULL.

Other Considerations When using ROLLUP

The following items list some behaviors specific to the MySQL implementation of ROLLUP.

When you use ROLLUP, you cannot also use an ORDER BY clause to sort the results. In other words, ROLLUP and ORDER BY are mutually exclusive. However, you still have some control over sort order. GROUP BY in MySQL sorts results, and you can use explicit ASC and DESC keywords with columns named in the GROUP BY list to specify sort order for individual columns. (The higher-level summary rows added by ROLLUP still appear after the rows from which they are calculated, regardless of the sort order.)

LIMIT can be used to restrict the number of rows returned to the client. LIMIT is applied after ROLLUP, so the limit applies against the extra rows added by ROLLUP. For example:

mysql> SELECT year, country, product, SUM(profit)
    -> FROM sales
    -> GROUP BY year, country, product WITH ROLLUP
    -> LIMIT 5;
+------+---------+------------+-------------+
| year | country | product    | SUM(profit) |
+------+---------+------------+-------------+
| 2000 | Finland | Computer   |        1500 |
| 2000 | Finland | Phone      |         100 |
| 2000 | Finland | NULL       |        1600 |
| 2000 | India   | Calculator |         150 |
| 2000 | India   | Computer   |        1200 |
+------+---------+------------+-------------+

Using LIMIT with ROLLUP may produce results that are more difficult to interpret, because you have less context for understanding the super-aggregate rows.

The NULL indicators in each super-aggregate row are produced when the row is sent to the client. The server looks at the columns named in the GROUP BY clause following the leftmost one that has changed value. For any column in the result set with a name that is a lexical match to any of those names, its value is set to NULL. (If you specify grouping columns by column number, the server identifies which columns to set to NULL by number.)

Because the NULL values in the super-aggregate rows are placed into the result set at such a late stage in query processing, you cannot test them as NULL values within the query itself. For example, you cannot add HAVING product IS NULL to the query to eliminate from the output all but the super-aggregate rows.

On the other hand, the NULL values do appear as NULL on the client side and can be tested as such using any MySQL client programming interface.

MySQL permits a column that does not appear in the GROUP BY list to be named in the select list. In this case, the server is free to choose any value from this nonaggregated column in summary rows, and this includes the extra rows added by WITH ROLLUP. For example, in the following query, country is a nonaggregated column that does not appear in the GROUP BY list and values chosen for this column are indeterminate:

mysql> SELECT year, country, SUM(profit)
    -> FROM sales GROUP BY year WITH ROLLUP;
+------+---------+-------------+
| year | country | SUM(profit) |
+------+---------+-------------+
| 2000 | India   |        4525 |
| 2001 | USA     |        3010 |
| NULL | USA     |        7535 |
+------+---------+-------------+

This behavior occurs if the ONLY_FULL_GROUP_BY SQL mode is not enabled. If that mode is enabled, the server rejects the query as illegal because country is not listed in the GROUP BY clause. For more information about nonaggregated columns and GROUP BY, see Section 12.17.3, “MySQL Handling of GROUP BY”.

12.17.3 MySQL Handling of GROUP BY

In standard SQL, a query that includes a GROUP BY clause cannot refer to nonaggregated columns in the select list that are not named in the GROUP BY clause. For example, this query is illegal in standard SQL because the name column in the select list does not appear in the GROUP BY:

SELECT o.custid, c.name, MAX(o.payment)
  FROM orders AS o, customers AS c
  WHERE o.custid = c.custid
  GROUP BY o.custid;

For the query to be legal, the name column must be omitted from the select list or named in the GROUP BY clause.

MySQL extends the use of GROUP BY so that the select list can refer to nonaggregated columns not named in the GROUP BY clause. This means that the preceding query is legal in MySQL. You can use this feature to get better performance by avoiding unnecessary column sorting and grouping. However, this is useful primarily when all values in each nonaggregated column not named in the GROUP BY are the same for each group. The server is free to choose any value from each group, so unless they are the same, the values chosen are indeterminate. Furthermore, the selection of values from each group cannot be influenced by adding an ORDER BY clause. Sorting of the result set occurs after values have been chosen, and ORDER BY does not affect which values within each group the server chooses.

A similar MySQL extension applies to the HAVING clause. In standard SQL, a query that includes a GROUP BY clause cannot refer to nonaggregated columns in the HAVING clause that are not named in the GROUP BY clause. A MySQL extension permits references to such columns to simplify calculations. This extension assumes that the nongrouped columns will have the same group-wise values. Otherwise, the result is indeterminate.

To disable the MySQL GROUP BY extension, enable the ONLY_FULL_GROUP_BY SQL mode. This enables standard SQL behavior: Columns not named in the GROUP BY clause cannot be used in the select list or HAVING clause unless enclosed in an aggregate function.

ONLY_FULL_GROUP_BY also affects use of aliases in the HAVING clauses. For example, the following query returns name values that occur only once in table orders:

SELECT name, COUNT(name) FROM orders
  GROUP BY name
  HAVING COUNT(name) = 1;

MySQL extends this behavior to permit the use of an alias in the HAVING clause for the aggregated column:

SELECT name, COUNT(name) AS c FROM orders
  GROUP BY name
  HAVING c = 1;

Enabling ONLY_FULL_GROUP_BY disables this MySQL extension and a non-grouping field 'c' is used in HAVING clause error occurs because the column c in the HAVING clause is not enclosed in an aggregate function (instead, it is an aggregate function).

The select list extension also applies to ORDER BY. That is, you can refer to nonaggregated columns in the ORDER BY clause that do not appear in the GROUP BY clause. (However, as mentioned previously, ORDER BY does not affect which values are chosen from nonaggregated columns; it only sorts them after they have been chosen.) This extension does not apply if the ONLY_FULL_GROUP_BY SQL mode is enabled.

In some cases, you can use MIN() and MAX() to obtain a specific column value even if it is not unique. If the sort column contains integers no larger than 6 digits, the following query gives the value of column from the row containing the smallest sort value:

SUBSTR(MIN(CONCAT(LPAD(sort,6,'0'),column)),7)

See Section 3.6.4, “The Rows Holding the Group-wise Maximum of a Certain Column”.

If you are trying to follow standard SQL, you cannot use expressions in GROUP BY clauses. As a workaround, use an alias for the expression:

SELECT id, FLOOR(value/100) AS val
  FROM tbl_name
  GROUP BY id, val;

MySQL permits expressions in GROUP BY clauses, so the alias is unnecessary:

SELECT id, FLOOR(value/100)
  FROM tbl_name
  GROUP BY id, FLOOR(value/100);

12.18 Precision Math

MySQL 5.5 provides support for precision math: numeric value handling that results in extremely accurate results and a high degree control over invalid values. Precision math is based on these two features:

  • SQL modes that control how strict the server is about accepting or rejecting invalid data.

  • The MySQL library for fixed-point arithmetic.

These features have several implications for numeric operations and provide a high degree of compliance with standard SQL:

  • Precise calculations: For exact-value numbers, calculations do not introduce floating-point errors. Instead, exact precision is used. For example, MySQL treats a number such as .0001 as an exact value rather than as an approximation, and summing it 10,000 times produces a result of exactly 1, not a value that is merely close to 1.

  • Well-defined rounding behavior: For exact-value numbers, the result of ROUND() depends on its argument, not on environmental factors such as how the underlying C library works.

  • Platform independence: Operations on exact numeric values are the same across different platforms such as Windows and Unix.

  • Control over handling of invalid values: Overflow and division by zero are detectable and can be treated as errors. For example, you can treat a value that is too large for a column as an error rather than having the value truncated to lie within the range of the column's data type. Similarly, you can treat division by zero as an error rather than as an operation that produces a result of NULL. The choice of which approach to take is determined by the setting of the server SQL mode.

The following discussion covers several aspects of how precision math works, including possible incompatibilities with older applications. At the end, some examples are given that demonstrate how MySQL 5.5 handles numeric operations precisely. For information about controlling the SQL mode, see Section 5.1.7, “Server SQL Modes”.

12.18.1 Types of Numeric Values

The scope of precision math for exact-value operations includes the exact-value data types (integer and DECIMAL types) and exact-value numeric literals. Approximate-value data types and numeric literals are handled as floating-point numbers.

Exact-value numeric literals have an integer part or fractional part, or both. They may be signed. Examples: 1, .2, 3.4, -5, -6.78, +9.10.

Approximate-value numeric literals are represented in scientific notation with a mantissa and exponent. Either or both parts may be signed. Examples: 1.2E3, 1.2E-3, -1.2E3, -1.2E-3.

Two numbers that look similar may be treated differently. For example, 2.34 is an exact-value (fixed-point) number, whereas 2.34E0 is an approximate-value (floating-point) number.

The DECIMAL data type is a fixed-point type and calculations are exact. In MySQL, the DECIMAL type has several synonyms: NUMERIC, DEC, FIXED. The integer types also are exact-value types.

The FLOAT and DOUBLE data types are floating-point types and calculations are approximate. In MySQL, types that are synonymous with FLOAT or DOUBLE are DOUBLE PRECISION and REAL.

12.18.2 DECIMAL Data Type Characteristics

This section discusses the characteristics of the DECIMAL data type (and its synonyms) in MySQL 5.5, with particular regard to the following topics:

  • Maximum number of digits

  • Storage format

  • Storage requirements

  • The nonstandard MySQL extension to the upper range of DECIMAL columns

Possible incompatibilities with applications that are written for older versions of MySQL (prior to 5.0.3) are noted throughout this section.

The declaration syntax for a DECIMAL column is DECIMAL(M,D). The ranges of values for the arguments in MySQL 5.5 are as follows:

  • M is the maximum number of digits (the precision). It has a range of 1 to 65. (Older versions of MySQL permitted a range of 1 to 254.)

  • D is the number of digits to the right of the decimal point (the scale). It has a range of 0 to 30 and must be no larger than M.

The maximum value of 65 for M means that calculations on DECIMAL values are accurate up to 65 digits. This limit of 65 digits of precision also applies to exact-value numeric literals, so the maximum range of such literals differs from before. (In older versions of MySQL, decimal values could have up to 254 digits. However, calculations were done using floating-point and thus were approximate, not exact.)

Values for DECIMAL columns in MySQL 5.5 are stored using a binary format that packs nine decimal digits into 4 bytes. The storage requirements for the integer and fractional parts of each value are determined separately. Each multiple of nine digits requires 4 bytes, and any remaining digits left over require some fraction of 4 bytes. The storage required for remaining digits is given by the following table.

Leftover DigitsNumber of Bytes
00
1–21
3–42
5–63
7–94

For example, a DECIMAL(18,9) column has nine digits on either side of the decimal point, so the integer part and the fractional part each require 4 bytes. A DECIMAL(20,6) column has fourteen integer digits and six fractional digits. The integer digits require four bytes for nine of the digits and 3 bytes for the remaining five digits. The six fractional digits require 3 bytes.

Unlike some older versions of MySQL, DECIMAL columns in MySQL 5.5 do not store a leading + character or - character or leading 0 digits. If you insert +0003.1 into a DECIMAL(5,1) column, it is stored as 3.1. For negative numbers, a literal - character is not stored. Applications that rely on the older behavior must be modified to account for this change.

DECIMAL columns in MySQL 5.5 do not permit values larger than the range implied by the column definition. For example, a DECIMAL(3,0) column supports a range of -999 to 999. A DECIMAL(M,D) column permits at most M - D digits to the left of the decimal point. This is not compatible with applications relying on older versions of MySQL that permitted storing an extra digit in lieu of a + sign.

The SQL standard requires that the precision of NUMERIC(M,D) be exactly M digits. For DECIMAL(M,D), the standard requires a precision of at least M digits but permits more. In MySQL, DECIMAL(M,D) and NUMERIC(M,D) are the same, and both have a precision of exactly M digits.

For a full explanation of the internal format of DECIMAL values, see the file strings/decimal.c in a MySQL source distribution. The format is explained (with an example) in the decimal2bin() function.

For more detailed information about porting applications that rely on the old treatment of the DECIMAL data type, see the MySQL 5.0 Reference Manual.

12.18.3 Expression Handling

With precision math, exact-value numbers are used as given whenever possible. For example, numbers in comparisons are used exactly as given without a change in value. In strict SQL mode, for INSERT into a column with an exact data type (DECIMAL or integer), a number is inserted with its exact value if it is within the column range. When retrieved, the value should be the same as what was inserted. (If strict SQL mode is not enabled, truncation for INSERT is permissible.)

Handling of a numeric expression depends on what kind of values the expression contains:

  • If any approximate values are present, the expression is approximate and is evaluated using floating-point arithmetic.

  • If no approximate values are present, the expression contains only exact values. If any exact value contains a fractional part (a value following the decimal point), the expression is evaluated using DECIMAL exact arithmetic and has a precision of 65 digits. The term exact is subject to the limits of what can be represented in binary. For example, 1.0/3.0 can be approximated in decimal notation as .333..., but not written as an exact number, so (1.0/3.0)*3.0 does not evaluate to exactly 1.0.

  • Otherwise, the expression contains only integer values. The expression is exact and is evaluated using integer arithmetic and has a precision the same as BIGINT (64 bits).

If a numeric expression contains any strings, they are converted to double-precision floating-point values and the expression is approximate.

Inserts into numeric columns are affected by the SQL mode, which is controlled by the sql_mode system variable. (See Section 5.1.7, “Server SQL Modes”.) The following discussion mentions strict mode (selected by the STRICT_ALL_TABLES or STRICT_TRANS_TABLES mode values) and ERROR_FOR_DIVISION_BY_ZERO. To turn on all restrictions, you can simply use TRADITIONAL mode, which includes both strict mode values and ERROR_FOR_DIVISION_BY_ZERO:

mysql> SET sql_mode='TRADITIONAL';

If a number is inserted into an exact type column (DECIMAL or integer), it is inserted with its exact value if it is within the column range.

If the value has too many digits in the fractional part, rounding occurs and a warning is generated. Rounding is done as described in Section 12.18.4, “Rounding Behavior”.

If the value has too many digits in the integer part, it is too large and is handled as follows:

  • If strict mode is not enabled, the value is truncated to the nearest legal value and a warning is generated.

  • If strict mode is enabled, an overflow error occurs.

Underflow is not detected, so underflow handling is undefined.

For inserts of strings into numeric columns, conversion from string to number is handled as follows if the string has nonnumeric contents:

  • A string that does not begin with a number cannot be used as a number and produces an error in strict mode, or a warning otherwise. This includes the empty string.

  • A string that begins with a number can be converted, but the trailing nonnumeric portion is truncated. If the truncated portion contains anything other than spaces, this produces an error in strict mode, or a warning otherwise.

By default, division by zero produces a result of NULL and no warning. By setting the SQL mode appropriately, division by zero can be restricted.

With the ERROR_FOR_DIVISION_BY_ZERO SQL mode enabled, MySQL handles division by zero differently:

  • If strict mode is not enabled, a warning occurs.

  • If strict mode is enabled, inserts and updates involving division by zero are prohibited, and an error occurs.

In other words, inserts and updates involving expressions that perform division by zero can be treated as errors, but this requires ERROR_FOR_DIVISION_BY_ZERO in addition to strict mode.

Suppose that we have this statement:

INSERT INTO t SET i = 1/0;

This is what happens for combinations of strict and ERROR_FOR_DIVISION_BY_ZERO modes.

sql_mode ValueResult
'' (Default)No warning, no error; i is set to NULL.
strictNo warning, no error; i is set to NULL.
ERROR_FOR_DIVISION_BY_ZEROWarning, no error; i is set to NULL.
strict,ERROR_FOR_DIVISION_BY_ZEROError condition; no row is inserted.

12.18.4 Rounding Behavior

This section discusses precision math rounding for the ROUND() function and for inserts into columns with exact-value types (DECIMAL and integer).

The ROUND() function rounds differently depending on whether its argument is exact or approximate:

  • For exact-value numbers, ROUND() uses the round half up rule: A value with a fractional part of .5 or greater is rounded up to the next integer if positive or down to the next integer if negative. (In other words, it is rounded away from zero.) A value with a fractional part less than .5 is rounded down to the next integer if positive or up to the next integer if negative.

  • For approximate-value numbers, the result depends on the C library. On many systems, this means that ROUND() uses the round to nearest even rule: A value with any fractional part is rounded to the nearest even integer.

The following example shows how rounding differs for exact and approximate values:

mysql> SELECT ROUND(2.5), ROUND(25E-1);
+------------+--------------+
| ROUND(2.5) | ROUND(25E-1) |
+------------+--------------+
| 3          |            2 |
+------------+--------------+

For inserts into a DECIMAL or integer column, the target is an exact data type, so rounding uses round half away from zero, regardless of whether the value to be inserted is exact or approximate:

mysql> CREATE TABLE t (d DECIMAL(10,0));
Query OK, 0 rows affected (0.00 sec)

mysql> INSERT INTO t VALUES(2.5),(2.5E0);
Query OK, 2 rows affected, 2 warnings (0.00 sec)
Records: 2  Duplicates: 0  Warnings: 2

mysql> SELECT d FROM t;
+------+
| d    |
+------+
| 3    |
| 3    |
+------+

12.18.5 Precision Math Examples

This section provides some examples that show precision math query results in MySQL 5.5. These examples demonstrate the principles described in Section 12.18.3, “Expression Handling”, and Section 12.18.4, “Rounding Behavior”.

Example 1. Numbers are used with their exact value as given when possible:

mysql> SELECT (.1 + .2) = .3;
+----------------+
| (.1 + .2) = .3 |
+----------------+
|              1 |
+----------------+

For floating-point values, results are inexact:

mysql> SELECT (.1E0 + .2E0) = .3E0;
+----------------------+
| (.1E0 + .2E0) = .3E0 |
+----------------------+
|                    0 |
+----------------------+

Another way to see the difference in exact and approximate value handling is to add a small number to a sum many times. Consider the following stored procedure, which adds .0001 to a variable 1,000 times.

CREATE PROCEDURE p ()
BEGIN
  DECLARE i INT DEFAULT 0;
  DECLARE d DECIMAL(10,4) DEFAULT 0;
  DECLARE f FLOAT DEFAULT 0;
  WHILE i < 10000 DO
    SET d = d + .0001;
    SET f = f + .0001E0;
    SET i = i + 1;
  END WHILE;
  SELECT d, f;
END;

The sum for both d and f logically should be 1, but that is true only for the decimal calculation. The floating-point calculation introduces small errors:

+--------+------------------+
| d      | f                |
+--------+------------------+
| 1.0000 | 0.99999999999991 |
+--------+------------------+

Example 2. Multiplication is performed with the scale required by standard SQL. That is, for two numbers X1 and X2 that have scale S1 and S2, the scale of the result is S1 + S2:

mysql> SELECT .01 * .01;
+-----------+
| .01 * .01 |
+-----------+
| 0.0001    |
+-----------+

Example 3. Rounding behavior for exact-value numbers is well-defined:

Rounding behavior (for example, with the ROUND() function) is independent of the implementation of the underlying C library, which means that results are consistent from platform to platform.

  • Rounding for exact-value columns (DECIMAL and integer) and exact-valued numbers uses the round half away from zero rule. Values with a fractional part of .5 or greater are rounded away from zero to the nearest integer, as shown here:

    mysql> SELECT ROUND(2.5), ROUND(-2.5);
    +------------+-------------+
    | ROUND(2.5) | ROUND(-2.5) |
    +------------+-------------+
    | 3          | -3          |
    +------------+-------------+
    
  • Rounding for floating-point values uses the C library, which on many systems uses the round to nearest even rule. Values with any fractional part on such systems are rounded to the nearest even integer:

    mysql> SELECT ROUND(2.5E0), ROUND(-2.5E0);
    +--------------+---------------+
    | ROUND(2.5E0) | ROUND(-2.5E0) |
    +--------------+---------------+
    |            2 |            -2 |
    +--------------+---------------+
    

Example 4. In strict mode, inserting a value that is out of range for a column causes an error, rather than truncation to a legal value.

When MySQL is not running in strict mode, truncation to a legal value occurs:

mysql> SET sql_mode='';
Query OK, 0 rows affected (0.00 sec)

mysql> CREATE TABLE t (i TINYINT);
Query OK, 0 rows affected (0.01 sec)

mysql> INSERT INTO t SET i = 128;
Query OK, 1 row affected, 1 warning (0.00 sec)

mysql> SELECT i FROM t;
+------+
| i    |
+------+
|  127 |
+------+
1 row in set (0.00 sec)

However, an error occurs if strict mode is in effect:

mysql> SET sql_mode='STRICT_ALL_TABLES';
Query OK, 0 rows affected (0.00 sec)

mysql> CREATE TABLE t (i TINYINT);
Query OK, 0 rows affected (0.00 sec)

mysql> INSERT INTO t SET i = 128;
ERROR 1264 (22003): Out of range value adjusted for column 'i' at row 1

mysql> SELECT i FROM t;
Empty set (0.00 sec)

Example 5: In strict mode and with ERROR_FOR_DIVISION_BY_ZERO set, division by zero causes an error, not a result of NULL.

In nonstrict mode, division by zero has a result of NULL:

mysql> SET sql_mode='';
Query OK, 0 rows affected (0.01 sec)

mysql> CREATE TABLE t (i TINYINT);
Query OK, 0 rows affected (0.00 sec)

mysql> INSERT INTO t SET i = 1 / 0;
Query OK, 1 row affected (0.00 sec)

mysql> SELECT i FROM t;
+------+
| i    |
+------+
| NULL |
+------+
1 row in set (0.03 sec)

However, division by zero is an error if the proper SQL modes are in effect:

mysql> SET sql_mode='STRICT_ALL_TABLES,ERROR_FOR_DIVISION_BY_ZERO';
Query OK, 0 rows affected (0.00 sec)

mysql> CREATE TABLE t (i TINYINT);
Query OK, 0 rows affected (0.00 sec)

mysql> INSERT INTO t SET i = 1 / 0;
ERROR 1365 (22012): Division by 0

mysql> SELECT i FROM t;
Empty set (0.01 sec)

Example 6. Exact-value literals are evaluated as exact values.

Prior to MySQL 5.0.3, exact-value and approximate-value literals both are evaluated as double-precision floating-point values:

mysql> SELECT VERSION();
+------------+
| VERSION()  |
+------------+
| 4.1.18-log |
+------------+
1 row in set (0.01 sec)

mysql> CREATE TABLE t SELECT 2.5 AS a, 25E-1 AS b;
Query OK, 1 row affected (0.07 sec)
Records: 1  Duplicates: 0  Warnings: 0

mysql> DESCRIBE t;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| a     | double(3,1) |      |     | 0.0     |       |
| b     | double      |      |     | 0       |       |
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.04 sec)

As of MySQL 5.0.3, the approximate-value literal is evaluated using floating point, but the exact-value literal is handled as DECIMAL:

mysql> SELECT VERSION();
+-----------------+
| VERSION()       |
+-----------------+
| 5.1.6-alpha-log |
+-----------------+
1 row in set (0.11 sec)

mysql> CREATE TABLE t SELECT 2.5 AS a, 25E-1 AS b;
Query OK, 1 row affected (0.01 sec)
Records: 1  Duplicates: 0  Warnings: 0

mysql> DESCRIBE t;
+-------+-----------------------+------+-----+---------+-------+
| Field | Type                  | Null | Key | Default | Extra |
+-------+-----------------------+------+-----+---------+-------+
| a     | decimal(2,1) unsigned | NO   |     | 0.0     |       |
| b     | double                | NO   |     | 0       |       |
+-------+-----------------------+------+-----+---------+-------+
2 rows in set (0.01 sec)

Example 7. If the argument to an aggregate function is an exact numeric type, the result is also an exact numeric type, with a scale at least that of the argument.

Consider these statements:

mysql> CREATE TABLE t (i INT, d DECIMAL, f FLOAT);
mysql> INSERT INTO t VALUES(1,1,1);
mysql> CREATE TABLE y SELECT AVG(i), AVG(d), AVG(f) FROM t;

Before MySQL 5.0.3, the result is a double no matter the argument type:

mysql> DESCRIBE y;
+--------+--------------+------+-----+---------+-------+
| Field  | Type         | Null | Key | Default | Extra |
+--------+--------------+------+-----+---------+-------+
| AVG(i) | double(17,4) | YES  |     | NULL    |       |
| AVG(d) | double(17,4) | YES  |     | NULL    |       |
| AVG(f) | double       | YES  |     | NULL    |       |
+--------+--------------+------+-----+---------+-------+

As of MySQL 5.0.3, the result is a double only for the floating-point argument. For exact type arguments, the result is also an exact type:

mysql> DESCRIBE y;
+--------+---------------+------+-----+---------+-------+
| Field  | Type          | Null | Key | Default | Extra |
+--------+---------------+------+-----+---------+-------+
| AVG(i) | decimal(14,4) | YES  |     | NULL    |       |
| AVG(d) | decimal(14,4) | YES  |     | NULL    |       |
| AVG(f) | double        | YES  |     | NULL    |       |
+--------+---------------+------+-----+---------+-------+

The result is a double only for the floating-point argument. For exact type arguments, the result is also an exact type.